diff --git a/app/javascript/mastodon/features/compose/components/upload.js b/app/javascript/mastodon/features/compose/components/upload.js
index c2bf3b72e..cd9e08360 100644
--- a/app/javascript/mastodon/features/compose/components/upload.js
+++ b/app/javascript/mastodon/features/compose/components/upload.js
@@ -79,7 +79,7 @@ export default class Upload extends ImmutablePureComponent {
                     placeholder={intl.formatMessage(messages.description)}
                     type='text'
                     value={description}
-                    maxLength={140}
+                    maxLength={420}
                     onFocus={this.handleInputFocus}
                     onChange={this.handleInputChange}
                     onBlur={this.handleInputBlur}
diff --git a/app/models/media_attachment.rb b/app/models/media_attachment.rb
index 25e41c209..60380198b 100644
--- a/app/models/media_attachment.rb
+++ b/app/models/media_attachment.rb
@@ -59,7 +59,7 @@ class MediaAttachment < ApplicationRecord
   validates_attachment_size :file, less_than: 8.megabytes
 
   validates :account, presence: true
-  validates :description, length: { maximum: 140 }, if: :local?
+  validates :description, length: { maximum: 420 }, if: :local?
 
   scope :attached,   -> { where.not(status_id: nil) }
   scope :unattached, -> { where(status_id: nil) }
@@ -140,7 +140,7 @@ class MediaAttachment < ApplicationRecord
   end
 
   def prepare_description
-    self.description = description.strip[0...140] unless description.nil?
+    self.description = description.strip[0...420] unless description.nil?
   end
 
   def set_type_and_extension
diff --git a/spec/models/media_attachment_spec.rb b/spec/models/media_attachment_spec.rb
index f20698c45..9fce5bc4f 100644
--- a/spec/models/media_attachment_spec.rb
+++ b/spec/models/media_attachment_spec.rb
@@ -52,9 +52,9 @@ RSpec.describe MediaAttachment, type: :model do
 
   describe 'descriptions for remote attachments' do
     it 'are cut off at 140 characters' do
-      media = Fabricate(:media_attachment, description: 'foo' * 100, remote_url: 'http://example.com/blah.jpg')
+      media = Fabricate(:media_attachment, description: 'foo' * 1000, remote_url: 'http://example.com/blah.jpg')
 
-      expect(media.description.size).to be <= 140
+      expect(media.description.size).to be <= 420
     end
   end
 end