diff --git a/app/controllers/api/base_controller.rb b/app/controllers/api/base_controller.rb
index ac8de5fc0f4..2bf8e82db86 100644
--- a/app/controllers/api/base_controller.rb
+++ b/app/controllers/api/base_controller.rb
@@ -68,7 +68,7 @@ class Api::BaseController < ApplicationController
   end
 
   def require_user!
-    if current_user && !current_user.disabled?
+    if current_user && !current_user.disabled? && current_user.confirmed?
       set_user_activity
     elsif current_user
       render json: { error: 'Your login is currently disabled' }, status: 403
diff --git a/app/controllers/api/v1/accounts_controller.rb b/app/controllers/api/v1/accounts_controller.rb
index f711c467675..6e4084c4e33 100644
--- a/app/controllers/api/v1/accounts_controller.rb
+++ b/app/controllers/api/v1/accounts_controller.rb
@@ -1,14 +1,16 @@
 # frozen_string_literal: true
 
 class Api::V1::AccountsController < Api::BaseController
-  before_action -> { authorize_if_got_token! :read, :'read:accounts' }, except: [:follow, :unfollow, :block, :unblock, :mute, :unmute]
+  before_action -> { authorize_if_got_token! :read, :'read:accounts' }, except: [:create, :follow, :unfollow, :block, :unblock, :mute, :unmute]
   before_action -> { doorkeeper_authorize! :follow, :'write:follows' }, only: [:follow, :unfollow]
   before_action -> { doorkeeper_authorize! :follow, :'write:mutes' }, only: [:mute, :unmute]
   before_action -> { doorkeeper_authorize! :follow, :'write:blocks' }, only: [:block, :unblock]
+  before_action -> { doorkeeper_authorize! :write, :'write:accounts' }, only: [:create]
 
-  before_action :require_user!, except: [:show]
-  before_action :set_account
+  before_action :require_user!, except: [:show, :create]
+  before_action :set_account, except: [:create]
   before_action :check_account_suspension, only: [:show]
+  before_action :check_enabled_registrations, only: [:create]
 
   respond_to :json
 
@@ -16,6 +18,16 @@ class Api::V1::AccountsController < Api::BaseController
     render json: @account, serializer: REST::AccountSerializer
   end
 
+  def create
+    token    = AppSignUpService.new.call(doorkeeper_token.application, account_params)
+    response = Doorkeeper::OAuth::TokenResponse.new(token)
+
+    headers.merge!(response.headers)
+
+    self.response_body = Oj.dump(response.body)
+    self.status        = response.status
+  end
+
   def follow
     FollowService.new.call(current_user.account, @account, reblogs: truthy_param?(:reblogs))
 
@@ -62,4 +74,12 @@ class Api::V1::AccountsController < Api::BaseController
   def check_account_suspension
     gone if @account.suspended?
   end
+
+  def account_params
+    params.permit(:username, :email, :password, :agreement)
+  end
+
+  def check_enabled_registrations
+    forbidden if single_user_mode? || !Setting.open_registrations
+  end
 end
diff --git a/app/controllers/auth/confirmations_controller.rb b/app/controllers/auth/confirmations_controller.rb
index 7af9cbe81db..c28c7471c0d 100644
--- a/app/controllers/auth/confirmations_controller.rb
+++ b/app/controllers/auth/confirmations_controller.rb
@@ -6,9 +6,9 @@ class Auth::ConfirmationsController < Devise::ConfirmationsController
   before_action :set_body_classes
   before_action :set_user, only: [:finish_signup]
 
-  # GET/PATCH /users/:id/finish_signup
   def finish_signup
     return unless request.patch? && params[:user]
+
     if @user.update(user_params)
       @user.skip_reconfirmation!
       bypass_sign_in(@user)
@@ -31,4 +31,12 @@ class Auth::ConfirmationsController < Devise::ConfirmationsController
   def user_params
     params.require(:user).permit(:email)
   end
+
+  def after_confirmation_path_for(_resource_name, user)
+    if user.created_by_application && truthy_param?(:redirect_to_app)
+      user.created_by_application.redirect_uri
+    else
+      super
+    end
+  end
 end
diff --git a/app/controllers/auth/registrations_controller.rb b/app/controllers/auth/registrations_controller.rb
index 088832be3c5..f2a832542fe 100644
--- a/app/controllers/auth/registrations_controller.rb
+++ b/app/controllers/auth/registrations_controller.rb
@@ -26,6 +26,7 @@ class Auth::RegistrationsController < Devise::RegistrationsController
 
     resource.locale      = I18n.locale
     resource.invite_code = params[:invite_code] if resource.invite_code.blank?
+    resource.agreement   = true
 
     resource.build_account if resource.account.nil?
   end
diff --git a/app/models/user.rb b/app/models/user.rb
index 44e0d11139c..77e48ed4b9d 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -36,6 +36,7 @@
 #  invite_id                 :bigint(8)
 #  remember_token            :string
 #  chosen_languages          :string           is an Array
+#  created_by_application_id :bigint(8)
 #
 
 class User < ApplicationRecord
@@ -66,6 +67,7 @@ class User < ApplicationRecord
 
   belongs_to :account, inverse_of: :user
   belongs_to :invite, counter_cache: :uses, optional: true
+  belongs_to :created_by_application, class_name: 'Doorkeeper::Application', optional: true
   accepts_nested_attributes_for :account
 
   has_many :applications, class_name: 'Doorkeeper::Application', as: :owner
@@ -74,6 +76,7 @@ class User < ApplicationRecord
   validates :locale, inclusion: I18n.available_locales.map(&:to_s), if: :locale?
   validates_with BlacklistedEmailValidator, if: :email_changed?
   validates_with EmailMxValidator, if: :validate_email_dns?
+  validates :agreement, acceptance: { allow_nil: false, accept: [true, 'true', '1'] }, on: :create
 
   scope :recent, -> { order(id: :desc) }
   scope :admins, -> { where(admin: true) }
@@ -294,7 +297,7 @@ class User < ApplicationRecord
       end
 
     if resource.blank?
-      resource = new(email: attributes[:email])
+      resource = new(email: attributes[:email], agreement: true)
       if Devise.check_at_sign && !resource[:email].index('@')
         resource[:email] = Rpam2.getenv(resource.find_pam_service, attributes[:email], attributes[:password], 'email', false)
         resource[:email] = "#{attributes[:email]}@#{resource.find_pam_suffix}" unless resource[:email]
@@ -307,7 +310,7 @@ class User < ApplicationRecord
     resource = joins(:account).find_by(accounts: { username: attributes[Devise.ldap_uid.to_sym].first })
 
     if resource.blank?
-      resource = new(email: attributes[:mail].first, account_attributes: { username: attributes[Devise.ldap_uid.to_sym].first })
+      resource = new(email: attributes[:mail].first, agreement: true, account_attributes: { username: attributes[Devise.ldap_uid.to_sym].first })
       resource.ldap_setup(attributes)
     end
 
diff --git a/app/services/app_sign_up_service.rb b/app/services/app_sign_up_service.rb
new file mode 100644
index 00000000000..1878587e809
--- /dev/null
+++ b/app/services/app_sign_up_service.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class AppSignUpService < BaseService
+  def call(app, params)
+    return unless allowed_registrations?
+
+    user_params    = params.slice(:email, :password, :agreement)
+    account_params = params.slice(:username)
+    user           = User.create!(user_params.merge(created_by_application: app, password_confirmation: user_params[:password], account_attributes: account_params))
+
+    Doorkeeper::AccessToken.create!(application: app,
+                                    resource_owner_id: user.id,
+                                    scopes: app.scopes,
+                                    expires_in: Doorkeeper.configuration.access_token_expires_in,
+                                    use_refresh_token: Doorkeeper.configuration.refresh_token_enabled?)
+  end
+
+  private
+
+  def allowed_registrations?
+    Setting.open_registrations && !Rails.configuration.x.single_user_mode
+  end
+end
diff --git a/app/views/user_mailer/confirmation_instructions.html.haml b/app/views/user_mailer/confirmation_instructions.html.haml
index 1f088a16f4f..f75f7529a8f 100644
--- a/app/views/user_mailer/confirmation_instructions.html.haml
+++ b/app/views/user_mailer/confirmation_instructions.html.haml
@@ -55,8 +55,12 @@
                             %tbody
                               %tr
                                 %td.button-primary
-                                  = link_to confirmation_url(@resource, confirmation_token: @token) do
-                                    %span= t 'devise.mailer.confirmation_instructions.action'
+                                  - if @resource.created_by_application
+                                    = link_to confirmation_url(@resource, confirmation_token: @token, redirect_to_app: 'true') do
+                                      %span= t 'devise.mailer.confirmation_instructions.action_with_app', app: @resource.created_by_application.name
+                                  - else
+                                    = link_to confirmation_url(@resource, confirmation_token: @token) do
+                                      %span= t 'devise.mailer.confirmation_instructions.action'
 
 %table.email-table{ cellspacing: 0, cellpadding: 0 }
   %tbody
diff --git a/app/views/user_mailer/confirmation_instructions.text.erb b/app/views/user_mailer/confirmation_instructions.text.erb
index e01eecb27e2..65b4626c669 100644
--- a/app/views/user_mailer/confirmation_instructions.text.erb
+++ b/app/views/user_mailer/confirmation_instructions.text.erb
@@ -4,7 +4,7 @@
 
 <%= t 'devise.mailer.confirmation_instructions.explanation', host: site_hostname %>
 
-=> <%= confirmation_url(@resource, confirmation_token: @token) %>
+=> <%= confirmation_url(@resource, confirmation_token: @token, redirect_to_app: @resource.created_by_application ? 'true' : nil) %>
 
 <%= strip_tags(t('devise.mailer.confirmation_instructions.extra_html', terms_path: about_more_url, policy_path: terms_url)) %>
 
diff --git a/config/initializers/rack_attack.rb b/config/initializers/rack_attack.rb
index 8756b8fbf0e..35302e37b1b 100644
--- a/config/initializers/rack_attack.rb
+++ b/config/initializers/rack_attack.rb
@@ -57,6 +57,10 @@ class Rack::Attack
     req.authenticated_user_id if req.post? && req.path.start_with?('/api/v1/media')
   end
 
+  throttle('throttle_api_sign_up', limit: 5, period: 30.minutes) do |req|
+    req.ip if req.post? && req.path == '/api/v1/accounts'
+  end
+
   throttle('protected_paths', limit: 25, period: 5.minutes) do |req|
     req.ip if req.post? && req.path =~ PROTECTED_PATHS_REGEX
   end
diff --git a/config/locales/devise.en.yml b/config/locales/devise.en.yml
index 20938e47b64..bd0642b2516 100644
--- a/config/locales/devise.en.yml
+++ b/config/locales/devise.en.yml
@@ -18,6 +18,7 @@ en:
     mailer:
       confirmation_instructions:
         action: Verify email address
+        action_with_app: Confirm and return to %{app}
         explanation: You have created an account on %{host} with this email address. You are one click away from activating it. If this wasn't you, please ignore this email.
         extra_html: Please also check out <a href="%{terms_path}">the rules of the instance</a> and <a href="%{policy_path}">our terms of service</a>.
         subject: 'Mastodon: Confirmation instructions for %{instance}'
diff --git a/config/routes.rb b/config/routes.rb
index 7723a08af97..808bb5acd6c 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -336,7 +336,7 @@ Rails.application.routes.draw do
         resources :relationships, only: :index
       end
 
-      resources :accounts, only: [:show] do
+      resources :accounts, only: [:create, :show] do
         resources :statuses, only: :index, controller: 'accounts/statuses'
         resources :followers, only: :index, controller: 'accounts/follower_accounts'
         resources :following, only: :index, controller: 'accounts/following_accounts'
diff --git a/db/migrate/20181219235220_add_created_by_application_id_to_users.rb b/db/migrate/20181219235220_add_created_by_application_id_to_users.rb
new file mode 100644
index 00000000000..17ce900af30
--- /dev/null
+++ b/db/migrate/20181219235220_add_created_by_application_id_to_users.rb
@@ -0,0 +1,8 @@
+class AddCreatedByApplicationIdToUsers < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_reference :users, :created_by_application, foreign_key: { to_table: 'oauth_applications', on_delete: :nullify }, index: false
+    add_index :users, :created_by_application_id, algorithm: :concurrently
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 51a7b5e7498..e47960b1693 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2018_12_13_185533) do
+ActiveRecord::Schema.define(version: 2018_12_19_235220) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -637,8 +637,10 @@ ActiveRecord::Schema.define(version: 2018_12_13_185533) do
     t.bigint "invite_id"
     t.string "remember_token"
     t.string "chosen_languages", array: true
+    t.bigint "created_by_application_id"
     t.index ["account_id"], name: "index_users_on_account_id"
     t.index ["confirmation_token"], name: "index_users_on_confirmation_token", unique: true
+    t.index ["created_by_application_id"], name: "index_users_on_created_by_application_id"
     t.index ["email"], name: "index_users_on_email", unique: true
     t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
   end
@@ -730,6 +732,7 @@ ActiveRecord::Schema.define(version: 2018_12_13_185533) do
   add_foreign_key "subscriptions", "accounts", name: "fk_9847d1cbb5", on_delete: :cascade
   add_foreign_key "users", "accounts", name: "fk_50500f500d", on_delete: :cascade
   add_foreign_key "users", "invites", on_delete: :nullify
+  add_foreign_key "users", "oauth_applications", column: "created_by_application_id", on_delete: :nullify
   add_foreign_key "web_push_subscriptions", "oauth_access_tokens", column: "access_token_id", on_delete: :cascade
   add_foreign_key "web_push_subscriptions", "users", on_delete: :cascade
   add_foreign_key "web_settings", "users", name: "fk_11910667b2", on_delete: :cascade
diff --git a/lib/mastodon/accounts_cli.rb b/lib/mastodon/accounts_cli.rb
index b219682232c..bbda244eade 100644
--- a/lib/mastodon/accounts_cli.rb
+++ b/lib/mastodon/accounts_cli.rb
@@ -73,7 +73,7 @@ module Mastodon
     def create(username)
       account  = Account.new(username: username)
       password = SecureRandom.hex
-      user     = User.new(email: options[:email], password: password, admin: options[:role] == 'admin', moderator: options[:role] == 'moderator', confirmed_at: Time.now.utc)
+      user     = User.new(email: options[:email], password: password, agreement: true, admin: options[:role] == 'admin', moderator: options[:role] == 'moderator', confirmed_at: options[:confirmed] ? Time.now.utc : nil)
 
       if options[:reattach]
         account = Account.find_local(username) || Account.new(username: username)
diff --git a/spec/controllers/api/v1/accounts_controller_spec.rb b/spec/controllers/api/v1/accounts_controller_spec.rb
index c506fb5f01a..f5f65c000e5 100644
--- a/spec/controllers/api/v1/accounts_controller_spec.rb
+++ b/spec/controllers/api/v1/accounts_controller_spec.rb
@@ -19,6 +19,40 @@ RSpec.describe Api::V1::AccountsController, type: :controller do
     end
   end
 
+  describe 'POST #create' do
+    let(:app) { Fabricate(:application) }
+    let(:token) { Doorkeeper::AccessToken.find_or_create_for(app, nil, 'read write', nil, false) }
+    let(:agreement) { nil }
+
+    before do
+      post :create, params: { username: 'test', password: '12345678', email: 'hello@world.tld', agreement: agreement }
+    end
+
+    context 'given truthy agreement' do
+      let(:agreement) { 'true' }
+
+      it 'returns http success' do
+        expect(response).to have_http_status(200)
+      end
+
+      it 'returns a new access token as JSON' do
+        expect(body_as_json[:access_token]).to_not be_blank
+      end
+
+      it 'creates a user' do
+        user = User.find_by(email: 'hello@world.tld')
+        expect(user).to_not be_nil
+        expect(user.created_by_application_id).to eq app.id
+      end
+    end
+
+    context 'given no agreement' do
+      it 'returns http unprocessable entity' do
+        expect(response).to have_http_status(422)
+      end
+    end
+  end
+
   describe 'GET #show' do
     let(:scopes) { 'read:accounts' }
 
diff --git a/spec/fabricators/user_fabricator.rb b/spec/fabricators/user_fabricator.rb
index 7dfbdb52d77..8f59565016d 100644
--- a/spec/fabricators/user_fabricator.rb
+++ b/spec/fabricators/user_fabricator.rb
@@ -3,4 +3,5 @@ Fabricator(:user) do
   email        { sequence(:email) { |i| "#{i}#{Faker::Internet.email}" } }
   password     "123456789"
   confirmed_at { Time.zone.now }
+  agreement    true
 end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index c829195972e..856254ce4b6 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -106,19 +106,19 @@ RSpec.describe User, type: :model do
     end
 
     it 'should allow a non-blacklisted user to be created' do
-      user = User.new(email: 'foo@example.com', account: account, password: password)
+      user = User.new(email: 'foo@example.com', account: account, password: password, agreement: true)
 
       expect(user.valid?).to be_truthy
     end
 
     it 'should not allow a blacklisted user to be created' do
-      user = User.new(email: 'foo@mvrht.com', account: account, password: password)
+      user = User.new(email: 'foo@mvrht.com', account: account, password: password, agreement: true)
 
       expect(user.valid?).to be_falsey
     end
 
     it 'should not allow a subdomain blacklisted user to be created' do
-      user = User.new(email: 'foo@mvrht.com.topdomain.tld', account: account, password: password)
+      user = User.new(email: 'foo@mvrht.com.topdomain.tld', account: account, password: password, agreement: true)
 
       expect(user.valid?).to be_falsey
     end
@@ -210,17 +210,17 @@ RSpec.describe User, type: :model do
     end
 
     it 'should not allow a user to be created unless they are whitelisted' do
-      user = User.new(email: 'foo@example.com', account: account, password: password)
+      user = User.new(email: 'foo@example.com', account: account, password: password, agreement: true)
       expect(user.valid?).to be_falsey
     end
 
     it 'should allow a user to be created if they are whitelisted' do
-      user = User.new(email: 'foo@mastodon.space', account: account, password: password)
+      user = User.new(email: 'foo@mastodon.space', account: account, password: password, agreement: true)
       expect(user.valid?).to be_truthy
     end
 
     it 'should not allow a user with a whitelisted top domain as subdomain in their email address to be created' do
-      user = User.new(email: 'foo@mastodon.space.userdomain.com', account: account, password: password)
+      user = User.new(email: 'foo@mastodon.space.userdomain.com', account: account, password: password, agreement: true)
       expect(user.valid?).to be_falsey
     end
 
@@ -242,7 +242,7 @@ RSpec.describe User, type: :model do
 
   it_behaves_like 'Settings-extended' do
     def create!
-      User.create!(account: Fabricate(:account), email: 'foo@mastodon.space', password: 'abcd1234')
+      User.create!(account: Fabricate(:account), email: 'foo@mastodon.space', password: 'abcd1234', agreement: true)
     end
 
     def fabricate
diff --git a/spec/services/app_sign_up_service_spec.rb b/spec/services/app_sign_up_service_spec.rb
new file mode 100644
index 00000000000..d480df34808
--- /dev/null
+++ b/spec/services/app_sign_up_service_spec.rb
@@ -0,0 +1,41 @@
+require 'rails_helper'
+
+RSpec.describe AppSignUpService, type: :service do
+  let(:app) { Fabricate(:application, scopes: 'read write') }
+  let(:good_params) { { username: 'alice', password: '12345678', email: 'good@email.com', agreement: true } }
+
+  subject { described_class.new }
+
+  describe '#call' do
+    it 'returns nil when registrations are closed' do
+      Setting.open_registrations = false
+      expect(subject.call(app, good_params)).to be_nil
+    end
+
+    it 'raises an error when params are missing' do
+      expect { subject.call(app, {}) }.to raise_error ActiveRecord::RecordInvalid
+    end
+
+    it 'creates an unconfirmed user with access token' do
+      access_token = subject.call(app, good_params)
+      expect(access_token).to_not be_nil
+      user = User.find_by(id: access_token.resource_owner_id)
+      expect(user).to_not be_nil
+      expect(user.confirmed?).to be false
+    end
+
+    it 'creates access token with the app\'s scopes' do
+      access_token = subject.call(app, good_params)
+      expect(access_token).to_not be_nil
+      expect(access_token.scopes.to_s).to eq 'read write'
+    end
+
+    it 'creates an account' do
+      access_token = subject.call(app, good_params)
+      expect(access_token).to_not be_nil
+      user = User.find_by(id: access_token.resource_owner_id)
+      expect(user).to_not be_nil
+      expect(user.account).to_not be_nil
+    end
+  end
+end