diff --git a/activestorage/CHANGELOG.md b/activestorage/CHANGELOG.md
index 75658a540f..8eb24c8791 100644
--- a/activestorage/CHANGELOG.md
+++ b/activestorage/CHANGELOG.md
@@ -1,3 +1,25 @@
+*   Add ability to use pre-defined variants.
+
+    ```ruby
+    class User < ActiveRecord::Base
+      has_one_attached :avatar do |attachable|
+        attachable.variant :thumb, resize: "100x100"
+        attachable.variant :medium, resize: "300x300", monochrome: true
+      end
+    end
+
+    class Gallery < ActiveRecord::Base
+      has_many_attached :photos do |attachable|
+        attachable.variant :thumb, resize: "100x100"
+        attachable.variant :medium, resize: "300x300", monochrome: true
+      end
+    end
+
+    <%= image_tag user.avatar.variant(:thumb) %>
+    ```
+
+    *fatkodima*
+
 *   After setting `config.active_storage.resolve_model_to_route = :rails_storage_proxy`
     `rails_blob_path` and `rails_representation_path` will generate proxy URLs by default.
 
diff --git a/activestorage/app/models/active_storage/attachment.rb b/activestorage/app/models/active_storage/attachment.rb
index 3c47fa2d29..4bde8d9bce 100644
--- a/activestorage/app/models/active_storage/attachment.rb
+++ b/activestorage/app/models/active_storage/attachment.rb
@@ -37,6 +37,19 @@ class ActiveStorage::Attachment < ActiveStorage::Record
     blob&.purge_later
   end
 
+  def variant(transformations)
+    case transformations
+    when Symbol
+      variant_name = transformations
+      transformations = variants.fetch(variant_name) do
+        record_model_name = record.to_model.model_name.name
+        raise ArgumentError, "Cannot find variant :#{variant_name} for #{record_model_name}##{name}"
+      end
+    end
+
+    blob.variant(transformations)
+  end
+
   private
     def analyze_blob_later
       blob.analyze_later unless blob.analyzed?
@@ -53,6 +66,10 @@ class ActiveStorage::Attachment < ActiveStorage::Record
     def dependent
       record.attachment_reflections[name]&.options[:dependent]
     end
+
+    def variants
+      record.attachment_reflections[name]&.variants
+    end
 end
 
 ActiveSupport.run_load_hooks :active_storage_attachment, ActiveStorage::Attachment
diff --git a/activestorage/lib/active_storage/attached/model.rb b/activestorage/lib/active_storage/attached/model.rb
index 85ddbb81b5..f0e5e6949a 100644
--- a/activestorage/lib/active_storage/attached/model.rb
+++ b/activestorage/lib/active_storage/attached/model.rb
@@ -83,6 +83,7 @@ module ActiveStorage
           { dependent: dependent, service_name: service },
           self
         )
+        yield reflection if block_given?
         ActiveRecord::Reflection.add_attachment_reflection(self, name, reflection)
       end
 
@@ -178,6 +179,7 @@ module ActiveStorage
           { dependent: dependent, service_name: service },
           self
         )
+        yield reflection if block_given?
         ActiveRecord::Reflection.add_attachment_reflection(self, name, reflection)
       end
 
diff --git a/activestorage/lib/active_storage/reflection.rb b/activestorage/lib/active_storage/reflection.rb
index ce248c88b5..5bec40d2d3 100644
--- a/activestorage/lib/active_storage/reflection.rb
+++ b/activestorage/lib/active_storage/reflection.rb
@@ -2,9 +2,19 @@
 
 module ActiveStorage
   module Reflection
+    class HasAttachedReflection < ActiveRecord::Reflection::MacroReflection #:nodoc:
+      def variant(name, transformations)
+        variants[name] = transformations
+      end
+
+      def variants
+        @variants ||= {}
+      end
+    end
+
     # Holds all the metadata about a has_one_attached attachment as it was
     # specified in the Active Record class.
-    class HasOneAttachedReflection < ActiveRecord::Reflection::MacroReflection #:nodoc:
+    class HasOneAttachedReflection < HasAttachedReflection #:nodoc:
       def macro
         :has_one_attached
       end
@@ -12,7 +22,7 @@ module ActiveStorage
 
     # Holds all the metadata about a has_many_attached attachment as it was
     # specified in the Active Record class.
-    class HasManyAttachedReflection < ActiveRecord::Reflection::MacroReflection #:nodoc:
+    class HasManyAttachedReflection < HasAttachedReflection #:nodoc:
       def macro
         :has_many_attached
       end
diff --git a/activestorage/test/models/attached/many_test.rb b/activestorage/test/models/attached/many_test.rb
index 5ecf2693cd..1cfdd3182b 100644
--- a/activestorage/test/models/attached/many_test.rb
+++ b/activestorage/test/models/attached/many_test.rb
@@ -628,6 +628,26 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     assert_match(/Cannot configure service :unknown for User#featured_photos/, error.message)
   end
 
+  test "creating variation by variation name" do
+    @user.highlights_with_variants.attach fixture_file_upload("racecar.jpg")
+    variant = @user.highlights_with_variants.first.variant(:thumb).processed
+
+    image = read_image(variant)
+    assert_equal "JPEG", image.type
+    assert_equal 100, image.width
+    assert_equal 67, image.height
+  end
+
+  test "raises error when unknown variant name is used" do
+    @user.highlights_with_variants.attach fixture_file_upload("racecar.jpg")
+
+    error = assert_raises ArgumentError do
+      @user.highlights_with_variants.first.variant(:unknown).processed
+    end
+
+    assert_match(/Cannot find variant :unknown for User#highlights_with_variants/, error.message)
+  end
+
   private
     def append_on_assign
       ActiveStorage.replace_on_assign_to_many, previous = false, ActiveStorage.replace_on_assign_to_many
diff --git a/activestorage/test/models/attached/one_test.rb b/activestorage/test/models/attached/one_test.rb
index dbdf547995..ac42dc5c65 100644
--- a/activestorage/test/models/attached/one_test.rb
+++ b/activestorage/test/models/attached/one_test.rb
@@ -609,4 +609,24 @@ class ActiveStorage::OneAttachedTest < ActiveSupport::TestCase
 
     assert_match(/Cannot configure service :unknown for User#featured_photo/, error.message)
   end
+
+  test "creating variation by variation name" do
+    @user.avatar_with_variants.attach fixture_file_upload("racecar.jpg")
+    variant = @user.avatar_with_variants.variant(:thumb).processed
+
+    image = read_image(variant)
+    assert_equal "JPEG", image.type
+    assert_equal 100, image.width
+    assert_equal 67, image.height
+  end
+
+  test "raises error when unknown variant name is used" do
+    @user.avatar_with_variants.attach fixture_file_upload("racecar.jpg")
+
+    error = assert_raises ArgumentError do
+      @user.avatar_with_variants.variant(:unknown).processed
+    end
+
+    assert_match(/Cannot find variant :unknown for User#avatar_with_variants/, error.message)
+  end
 end
diff --git a/activestorage/test/models/reflection_test.rb b/activestorage/test/models/reflection_test.rb
index d3f8b39cad..4e258eb8e9 100644
--- a/activestorage/test/models/reflection_test.rb
+++ b/activestorage/test/models/reflection_test.rb
@@ -12,6 +12,9 @@ class ActiveStorage::ReflectionTest < ActiveSupport::TestCase
 
     reflection = User.reflect_on_attachment(:cover_photo)
     assert_equal :local, reflection.options[:service_name]
+
+    reflection = User.reflect_on_attachment(:avatar_with_variants)
+    assert_instance_of Hash, reflection.variants
   end
 
   test "reflection on a singular attachment with the same name as an attachment on another model" do
@@ -28,13 +31,16 @@ class ActiveStorage::ReflectionTest < ActiveSupport::TestCase
 
     reflection = User.reflect_on_attachment(:vlogs)
     assert_equal :local, reflection.options[:service_name]
+
+    reflection = User.reflect_on_attachment(:highlights_with_variants)
+    assert_instance_of Hash, reflection.variants
   end
 
   test "reflecting on all attachments" do
     reflections = User.reflect_on_all_attachments.sort_by(&:name)
     assert_equal [ User ], reflections.collect(&:active_record).uniq
-    assert_equal %i[ avatar cover_photo highlights vlogs ], reflections.collect(&:name)
-    assert_equal %i[ has_one_attached has_one_attached has_many_attached has_many_attached ], reflections.collect(&:macro)
-    assert_equal [ :purge_later, false, :purge_later, false ], reflections.collect { |reflection| reflection.options[:dependent] }
+    assert_equal %i[ avatar avatar_with_variants cover_photo highlights highlights_with_variants vlogs ], reflections.collect(&:name)
+    assert_equal %i[ has_one_attached has_one_attached has_one_attached has_many_attached has_many_attached has_many_attached ], reflections.collect(&:macro)
+    assert_equal [ :purge_later, :purge_later, false, :purge_later, :purge_later, false ], reflections.collect { |reflection| reflection.options[:dependent] }
   end
 end
diff --git a/activestorage/test/test_helper.rb b/activestorage/test/test_helper.rb
index 11e3eac98e..e85c82ae99 100644
--- a/activestorage/test/test_helper.rb
+++ b/activestorage/test/test_helper.rb
@@ -119,9 +119,15 @@ class User < ActiveRecord::Base
 
   has_one_attached :avatar
   has_one_attached :cover_photo, dependent: false, service: :local
+  has_one_attached :avatar_with_variants do |attachable|
+    attachable.variant :thumb, resize: "100x100"
+  end
 
   has_many_attached :highlights
   has_many_attached :vlogs, dependent: false, service: :local
+  has_many_attached :highlights_with_variants do |attachable|
+    attachable.variant :thumb, resize: "100x100"
+  end
 end
 
 class Group < ActiveRecord::Base
diff --git a/guides/source/active_storage_overview.md b/guides/source/active_storage_overview.md
index 4ed259c2e2..09d376eadc 100644
--- a/guides/source/active_storage_overview.md
+++ b/guides/source/active_storage_overview.md
@@ -352,6 +352,22 @@ class User < ApplicationRecord
 end
 ```
 
+You can configure specific variants per attachment by calling the `variant` method on yielded attachable object:
+
+```ruby
+class User < ApplicationRecord
+  has_one_attached :avatar do |attachable|
+    attachable.variant :thumb, resize: "100x100"
+  end
+end
+```
+
+Call `avatar.variant(:thumb)` to get a thumb variant of an avatar:
+
+```ruby
+<%= image_tag user.avatar.variant(:thumb) %>
+```
+
 [`has_one_attached`]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/Model.html#method-i-has_one_attached
 [Attached::One#attach]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/One.html#method-i-attach
 [Attached::One#attached?]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/One.html#method-i-attached-3F
@@ -406,10 +422,21 @@ class Message < ApplicationRecord
 end
 ```
 
+Configuring specific variants is done the same way as `has_one_attached`, by calling the `variant` method on the yielded attachable object:
+
+```ruby
+class Message < ApplicationRecord
+  has_many_attached :images do |attachable|
+    attachable.variant :thumb, resize: "100x100"
+  end
+end
+```
+
 [`has_many_attached`]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/Model.html#method-i-has_many_attached
 [Attached::Many#attach]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/Many.html#method-i-attach
 [Attached::Many#attached?]: https://api.rubyonrails.org/classes/ActiveStorage/Attached/Many.html#method-i-attached-3F
 
+
 ### Attaching File/IO Objects
 
 Sometimes you need to attach a file that doesn’t arrive via an HTTP request.