diff --git a/app/controllers/api/v1/accounts/credentials_controller.rb b/app/controllers/api/v1/accounts/credentials_controller.rb
index 64b5cb747..5682bdb01 100644
--- a/app/controllers/api/v1/accounts/credentials_controller.rb
+++ b/app/controllers/api/v1/accounts/credentials_controller.rb
@@ -21,7 +21,7 @@ class Api::V1::Accounts::CredentialsController < Api::BaseController
   private
 
   def account_params
-    params.permit(:display_name, :note, :avatar, :header, :locked, :bot, :discoverable, fields_attributes: [:name, :value])
+    params.permit(:display_name, :note, :avatar, :header, :locked, :bot, :cat, :discoverable, fields_attributes: [:name, :value])
   end
 
   def user_settings_params
diff --git a/app/controllers/settings/profiles_controller.rb b/app/controllers/settings/profiles_controller.rb
index 0c15447a6..f78953828 100644
--- a/app/controllers/settings/profiles_controller.rb
+++ b/app/controllers/settings/profiles_controller.rb
@@ -20,7 +20,7 @@ class Settings::ProfilesController < Settings::BaseController
   private
 
   def account_params
-    params.require(:account).permit(:display_name, :note, :avatar, :header, :locked, :bot, :discoverable, fields_attributes: [:name, :value])
+    params.require(:account).permit(:display_name, :note, :avatar, :header, :locked, :bot, :cat, :discoverable, fields_attributes: [:name, :value])
   end
 
   def set_account
diff --git a/app/lib/activitypub/adapter.rb b/app/lib/activitypub/adapter.rb
index 2d6b87659..655c290a7 100644
--- a/app/lib/activitypub/adapter.rb
+++ b/app/lib/activitypub/adapter.rb
@@ -24,6 +24,7 @@ class ActivityPub::Adapter < ActiveModelSerializers::Adapter::Base
     voters_count: { 'toot' => 'http://joinmastodon.org/ns#', 'votersCount' => 'toot:votersCount' },
     olm: { 'toot' => 'http://joinmastodon.org/ns#', 'Device' => 'toot:Device', 'Ed25519Signature' => 'toot:Ed25519Signature', 'Ed25519Key' => 'toot:Ed25519Key', 'Curve25519Key' => 'toot:Curve25519Key', 'EncryptedMessage' => 'toot:EncryptedMessage', 'publicKeyBase64' => 'toot:publicKeyBase64', 'deviceId' => 'toot:deviceId', 'claim' => { '@type' => '@id', '@id' => 'toot:claim' }, 'fingerprintKey' => { '@type' => '@id', '@id' => 'toot:fingerprintKey' }, 'identityKey' => { '@type' => '@id', '@id' => 'toot:identityKey' }, 'devices' => { '@type' => '@id', '@id' => 'toot:devices' }, 'messageFranking' => 'toot:messageFranking', 'messageType' => 'toot:messageType', 'cipherText' => 'toot:cipherText' },
     suspended: { 'toot' => 'http://joinmastodon.org/ns#', 'suspended' => 'toot:suspended' },
+    is_cat: { 'isCat' => 'as:isCat' },
   }.freeze
 
   def self.default_key_transform
diff --git a/app/lib/formatter.rb b/app/lib/formatter.rb
index 7f217ae9f..95854b4b0 100644
--- a/app/lib/formatter.rb
+++ b/app/lib/formatter.rb
@@ -28,6 +28,7 @@ class Formatter
     unless status.local?
       html = reformat(raw_content)
       html = encode_custom_emojis(html, status.emojis, options[:autoplay]) if options[:custom_emojify]
+      html = nyaize(html) if options[:nyaize]
       return html.html_safe # rubocop:disable Rails/OutputSafety
     end
 
@@ -39,6 +40,7 @@ class Formatter
     html = encode_and_link_urls(html, linkable_accounts)
     html = encode_custom_emojis(html, status.emojis, options[:autoplay]) if options[:custom_emojify]
     html = simple_format(html, {}, sanitize: false)
+    html = nyaize(html) if options[:nyaize]
     html = html.delete("\n")
 
     html.html_safe # rubocop:disable Rails/OutputSafety
@@ -192,6 +194,10 @@ class Formatter
   end
   # rubocop:enable Metrics/BlockNesting
 
+  def nyaize(html)
+    html.gsub(/な/, "にゃ").gsub(/ナ/, "ニャ").gsub(/ﾅ/, "ﾆｬ").gsub(/[나-낳]/){|c|(c.ord + '냐'.ord - '나'.ord).chr}
+  end
+
   def rewrite(text, entities)
     text = text.to_s
 
diff --git a/app/models/account.rb b/app/models/account.rb
index e6cf03fa8..5f769a68f 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -41,6 +41,7 @@
 #  featured_collection_url       :string
 #  fields                        :jsonb
 #  actor_type                    :string
+#  cat                           :boolean          default(FALSE), not null
 #  discoverable                  :boolean
 #  also_known_as                 :string           is an Array
 #  silenced_at                   :datetime
diff --git a/app/models/status.rb b/app/models/status.rb
index b426f9d5b..00a1c7827 100644
--- a/app/models/status.rb
+++ b/app/models/status.rb
@@ -129,7 +129,10 @@ class Status < ApplicationRecord
                    ],
                    thread: { account: :account_stat }
 
-  delegate :domain, to: :account, prefix: true
+  delegate :domain,
+           :cat,
+           to: :account,
+           prefix: true
 
   REAL_TIME_WINDOW = 6.hours
 
diff --git a/app/serializers/activitypub/actor_serializer.rb b/app/serializers/activitypub/actor_serializer.rb
index 759ef30f9..3b4c4df0d 100644
--- a/app/serializers/activitypub/actor_serializer.rb
+++ b/app/serializers/activitypub/actor_serializer.rb
@@ -12,7 +12,7 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
   attributes :id, :type, :following, :followers,
              :inbox, :outbox, :featured, :featured_tags,
              :preferred_username, :name, :summary,
-             :url, :manually_approves_followers,
+             :url, :manually_approves_followers, :is_cat
              :discoverable
 
   has_one :public_key, serializer: ActivityPub::PublicKeySerializer
@@ -138,6 +138,10 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
     object.suspended? ? false : object.locked
   end
 
+  def is_cat
+    object.cat
+  end
+
   def virtual_tags
     object.suspended? ? [] : (object.emojis + object.tags)
   end
diff --git a/app/serializers/rest/account_serializer.rb b/app/serializers/rest/account_serializer.rb
index 189a62d0e..1b7946bcc 100644
--- a/app/serializers/rest/account_serializer.rb
+++ b/app/serializers/rest/account_serializer.rb
@@ -3,7 +3,7 @@
 class REST::AccountSerializer < ActiveModel::Serializer
   include RoutingHelper
 
-  attributes :id, :username, :acct, :display_name, :locked, :bot, :discoverable, :group, :created_at,
+  attributes :id, :username, :acct, :display_name, :locked, :bot, :cat, :discoverable, :group, :created_at,
              :note, :url, :avatar, :avatar_static, :header, :header_static,
              :followers_count, :following_count, :statuses_count, :last_status_at
 
diff --git a/app/serializers/rest/status_serializer.rb b/app/serializers/rest/status_serializer.rb
index bb6df90b7..7df28832a 100644
--- a/app/serializers/rest/status_serializer.rb
+++ b/app/serializers/rest/status_serializer.rb
@@ -71,7 +71,7 @@ class REST::StatusSerializer < ActiveModel::Serializer
   end
 
   def content
-    Formatter.instance.format(object)
+    Formatter.instance.format(object, nyaize: object.account.cat)
   end
 
   def url
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index 6afeb92d6..2621a07ae 100644
--- a/app/services/activitypub/process_account_service.rb
+++ b/app/services/activitypub/process_account_service.rb
@@ -112,6 +112,7 @@ class ActivityPub::ProcessAccountService < BaseService
     @account.followers_count   = followers_total_items if followers_total_items.present?
     @account.hide_collections  = following_private? || followers_private?
     @account.moved_to_account  = @json['movedTo'].present? ? moved_account : nil
+    @account.cat               = @json['isCat'] || false
   end
 
   def set_suspension!
diff --git a/app/views/admin/reports/_status.html.haml b/app/views/admin/reports/_status.html.haml
index fa15796d2..e03eb778a 100644
--- a/app/views/admin/reports/_status.html.haml
+++ b/app/views/admin/reports/_status.html.haml
@@ -4,12 +4,12 @@
   .batch-table__row__content
     .status__content><
       - if status.proper.spoiler_text.blank?
-        = Formatter.instance.format(status.proper, custom_emojify: true)
+        = Formatter.instance.format(status.proper, custom_emojify: true, nyaize: status.proper.account.cat)
       - else
         %details<
           %summary><
             %strong> Content warning: #{Formatter.instance.format_spoiler(status.proper)}
-          = Formatter.instance.format(status.proper, custom_emojify: true)
+          = Formatter.instance.format(status.proper, custom_emojify: true, nyaize: status.proper.account.cat)
 
     - unless status.proper.media_attachments.empty?
       - if status.proper.media_attachments.first.video?
diff --git a/app/views/notification_mailer/_status.html.haml b/app/views/notification_mailer/_status.html.haml
index 9b7e1b65c..06b970367 100644
--- a/app/views/notification_mailer/_status.html.haml
+++ b/app/views/notification_mailer/_status.html.haml
@@ -31,7 +31,7 @@
                                     = Formatter.instance.format_spoiler(status)
 
                               %div.auto-dir
-                                = Formatter.instance.format(status)
+                                = Formatter.instance.format(status, nyaize: status.account.cat)
 
                                 - if status.media_attachments.size > 0
                                   %p
diff --git a/app/views/settings/profiles/show.html.haml b/app/views/settings/profiles/show.html.haml
index 4885878f0..ce13ec2ac 100644
--- a/app/views/settings/profiles/show.html.haml
+++ b/app/views/settings/profiles/show.html.haml
@@ -29,6 +29,9 @@
   .fields-group
     = f.input :bot, as: :boolean, wrapper: :with_label, hint: t('simple_form.hints.defaults.bot')
 
+  .fields-group
+    = f.input :cat, as: :boolean, wrapper: :with_label, hint: t('simple_form.hints.defaults.cat')
+
   - if Setting.profile_directory
     .fields-group
       = f.input :discoverable, as: :boolean, wrapper: :with_label, hint: t('simple_form.hints.defaults.discoverable'), recommended: true
diff --git a/app/views/statuses/_detailed_status.html.haml b/app/views/statuses/_detailed_status.html.haml
index 4c879472d..48b824b47 100644
--- a/app/views/statuses/_detailed_status.html.haml
+++ b/app/views/statuses/_detailed_status.html.haml
@@ -21,7 +21,7 @@
         %span.p-summary> #{Formatter.instance.format_spoiler(status, autoplay: autoplay)}&nbsp;
         %button.status__content__spoiler-link= t('statuses.show_more')
     .e-content
-      = Formatter.instance.format(status, custom_emojify: true, autoplay: autoplay)
+      = Formatter.instance.format(status, custom_emojify: true, autoplay: autoplay, nyaize: status.account.cat)
       - if status.preloadable_poll
         = react_component :poll, disabled: true, poll: ActiveModelSerializers::SerializableResource.new(status.preloadable_poll, serializer: REST::PollSerializer, scope: current_user, scope_name: :current_user).as_json do
           = render partial: 'statuses/poll', locals: { status: status, poll: status.preloadable_poll, autoplay: autoplay }
diff --git a/app/views/statuses/_simple_status.html.haml b/app/views/statuses/_simple_status.html.haml
index 236948a84..0465440f7 100644
--- a/app/views/statuses/_simple_status.html.haml
+++ b/app/views/statuses/_simple_status.html.haml
@@ -30,7 +30,7 @@
         %span.p-summary> #{Formatter.instance.format_spoiler(status, autoplay: autoplay)}&nbsp;
         %button.status__content__spoiler-link= t('statuses.show_more')
     .e-content
-      = Formatter.instance.format(status, custom_emojify: true, autoplay: autoplay)
+      = Formatter.instance.format(status, custom_emojify: true, autoplay: autoplay, nyaize: status.account.cat)
       - if status.preloadable_poll
         = react_component :poll, disabled: true, poll: ActiveModelSerializers::SerializableResource.new(status.preloadable_poll, serializer: REST::PollSerializer, scope: current_user, scope_name: :current_user).as_json do
           = render partial: 'statuses/poll', locals: { status: status, poll: status.preloadable_poll, autoplay: autoplay }
diff --git a/config/locales/simple_form.en.yml b/config/locales/simple_form.en.yml
index 20c916560..3f6acbb14 100644
--- a/config/locales/simple_form.en.yml
+++ b/config/locales/simple_form.en.yml
@@ -25,6 +25,7 @@ en:
         autofollow: People who sign up through the invite will automatically follow you
         avatar: PNG, GIF or JPG. At most %{size}. Will be downscaled to %{dimensions}px
         bot: This account mainly performs automated actions and might not be monitored
+        cat: This account is Cat
         context: One or multiple contexts where the filter should apply
         current_password: For security purposes please enter the password of the current account
         current_username: To confirm, please enter the username of the current account
diff --git a/config/locales/simple_form.ja.yml b/config/locales/simple_form.ja.yml
index abe986acd..0584b8122 100644
--- a/config/locales/simple_form.ja.yml
+++ b/config/locales/simple_form.ja.yml
@@ -25,6 +25,7 @@ ja:
         autofollow: 招待から登録した人が自動的にあなたをフォローするようになります
         avatar: "%{size}までのPNG、GIF、JPGが利用可能です。%{dimensions}pxまで縮小されます"
         bot: このアカウントは主に自動で動作し、人が見ていない可能性があります
+        cat: このアカウントはCatです
         context: フィルターを適用する対象 (複数選択可)
         current_password: 現在のアカウントのパスワードを入力してください
         current_username: 確認のため、現在のアカウントのユーザー名を入力してください
diff --git a/db/migrate/20181122090024_add_cat_to_accounts.rb b/db/migrate/20181122090024_add_cat_to_accounts.rb
new file mode 100644
index 000000000..cfd4ea6d7
--- /dev/null
+++ b/db/migrate/20181122090024_add_cat_to_accounts.rb
@@ -0,0 +1,7 @@
+class AddCatToAccounts < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_column :accounts, :cat, :boolean, default: false, null: false
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 18bf1d4ca..34cdabff9 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -180,6 +180,7 @@ ActiveRecord::Schema.define(version: 2020_12_18_054746) do
     t.string "featured_collection_url"
     t.jsonb "fields"
     t.string "actor_type"
+    t.boolean "cat", default: false, null: false
     t.boolean "discoverable"
     t.string "also_known_as", array: true
     t.datetime "silenced_at"