diff --git a/actionpack/lib/action_dispatch/journey/gtg/transition_table.rb b/actionpack/lib/action_dispatch/journey/gtg/transition_table.rb
index 239135ec8af0d..14f8d14fc4c09 100644
--- a/actionpack/lib/action_dispatch/journey/gtg/transition_table.rb
+++ b/actionpack/lib/action_dispatch/journey/gtg/transition_table.rb
@@ -22,6 +22,18 @@ def initialize
           @memos           = Hash.new { |h, k| h[k] = [] }
         end
 
+        def freeze
+          return self if self.frozen?
+
+          @memos.default_proc = nil
+          @stdparam_states.freeze
+          @regexp_states.freeze
+          @string_states.freeze
+          @accepting.freeze
+          @memos.freeze
+          super
+        end
+
         def add_accepting(state)
           @accepting[state] = true
         end
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index 1497db7cd2366..158d609b1c8ab 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -351,12 +351,12 @@ class Railtie < Rails::Railtie # :nodoc:
       ActiveRecord.message_verifiers = app.message_verifiers
 
       use_legacy_signed_id_verifier = app.config.active_record.use_legacy_signed_id_verifier
-      legacy_options = { digest: "SHA256", serializer: JSON, url_safe: true }
+      legacy_options = { digest: "SHA256", serializer: JSON, url_safe: true }.freeze
 
       if use_legacy_signed_id_verifier == :generate_and_verify
-        app.message_verifiers.prepend { |salt| legacy_options if salt == "active_record/signed_id" }
+        app.message_verifiers.prepend(&ActiveSupport::Ractors.shareable_proc { |salt| legacy_options if salt == "active_record/signed_id" })
       elsif use_legacy_signed_id_verifier == :verify
-        app.message_verifiers.rotate { |salt| legacy_options if salt == "active_record/signed_id" }
+        app.message_verifiers.rotate(&ActiveSupport::Ractors.shareable_proc { |salt| legacy_options if salt == "active_record/signed_id" })
       elsif use_legacy_signed_id_verifier
         raise ArgumentError, "Unrecognized value for config.active_record.use_legacy_signed_id_verifier: #{use_legacy_signed_id_verifier.inspect}"
       end
diff --git a/activesupport/lib/active_support/key_generator.rb b/activesupport/lib/active_support/key_generator.rb
index 0cca53dd32970..e98cea16bc02c 100644
--- a/activesupport/lib/active_support/key_generator.rb
+++ b/activesupport/lib/active_support/key_generator.rb
@@ -62,9 +62,21 @@ def initialize(key_generator)
       @cache_keys = Concurrent::Map.new
     end
 
+    def freeze
+      @key = "_caching_key_generator_#{object_id}".to_sym
+      Ractor[@key] = @cache_keys
+      @cache_keys = nil
+      super
+    end
+
     # Returns a derived key suitable for use.
     def generate_key(*args)
-      @cache_keys[args.join("|")] ||= @key_generator.generate_key(*args)
+      cache_keys[args.join("|")] ||= @key_generator.generate_key(*args)
     end
+
+    private
+      def cache_keys
+        @cache_keys || Ractor[@key] ||= Concurrent::Map.new
+      end
   end
 end
diff --git a/activesupport/lib/active_support/ordered_options.rb b/activesupport/lib/active_support/ordered_options.rb
index d65f20181f458..bf04a0c8bef2e 100644
--- a/activesupport/lib/active_support/ordered_options.rb
+++ b/activesupport/lib/active_support/ordered_options.rb
@@ -100,6 +100,16 @@ def initialize(parent = nil)
       end
     end
 
+    def freeze
+      return self if frozen?
+
+      @own_keys = own_keys.dup.freeze
+      self.default_proc = nil
+      @parent.freeze
+      replace(to_h)
+      super
+    end
+
     def to_h
       @parent.to_h.merge(self)
     end
@@ -120,13 +130,17 @@ def pretty_print(pp)
       pp.pp_hash(to_h)
     end
 
-    alias_method :own_key?, :key?
-    private :own_key?
-
     def key?(key)
       super || @parent.key?(key)
     end
 
+    alias_method :_own_keys, :keys
+    private :_own_keys
+
+    def keys
+      @parent.keys | super
+    end
+
     def overridden?(key)
       !!(@parent && @parent.key?(key) && own_key?(key.to_sym))
     end
@@ -143,5 +157,14 @@ def each(&block)
       to_h.each(&block)
       self
     end
+
+    private
+      def own_key?(key)
+        own_keys.include?(key.to_sym)
+      end
+
+      def own_keys
+        @own_keys || _own_keys
+      end
   end
 end
diff --git a/activesupport/lib/active_support/testing/ractors_assertions.rb b/activesupport/lib/active_support/testing/ractors_assertions.rb
index 22242d67b9a41..144903de943e1 100644
--- a/activesupport/lib/active_support/testing/ractors_assertions.rb
+++ b/activesupport/lib/active_support/testing/ractors_assertions.rb
@@ -5,6 +5,18 @@ module Testing
     module RactorsAssertions # :nodoc: all
       private
         if RUBY_VERSION >= "4.0"
+          def on_ractor(*args, &block)
+            block = Ractor.shareable_proc(&block)
+
+            port = Ractor::Port.new
+
+            Ractor.new(port, block, args) do |port, block, args|
+              port.send block.call(*args)
+            end.join
+
+            port.receive
+          end
+
           def assert_ractor_make_shareable(obj)
             assert_nothing_raised { Ractor.make_shareable(obj) }
           end
@@ -13,6 +25,10 @@ def assert_ractor_shareable(obj)
             assert Ractor.shareable?(obj), "Expected #{obj.inspect} to be shareable, but it is not."
           end
         else
+          def on_ractor(*args)
+            yield(*args)
+          end
+
           def assert_ractor_make_shareable(obj)
             assert true
           end
diff --git a/activesupport/test/key_generator_test.rb b/activesupport/test/key_generator_test.rb
index 87cc7c8de0922..9d761624c2be1 100644
--- a/activesupport/test/key_generator_test.rb
+++ b/activesupport/test/key_generator_test.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative "abstract_unit"
+require "active_support/testing/ractors_assertions"
 
 begin
   require "openssl"
@@ -66,6 +67,8 @@ def setup
   end
 
   class CachingKeyGeneratorTest < ActiveSupport::TestCase
+    include ActiveSupport::Testing::RactorsAssertions
+
     def setup
       @secret    = SecureRandom.hex(64)
       @generator = ActiveSupport::KeyGenerator.new(@secret, iterations: 2)
@@ -100,5 +103,23 @@ def setup
 
       assert_not_equal derived_key, different_length_key
     end
+
+    test "CachingKeyGenerator can work across ractors" do
+      # OpenSSL::Digest are not Ractor-safe, but the fix is already merged upstream. This test can be updated
+      # to use our implementation once a version of Ruby ships with ruby/openssl@502bc6c
+      key_generator = Class.new(ActiveSupport::KeyGenerator) do
+        def generate_key(salt, key_size)
+          OpenSSL::PKCS5.pbkdf2_hmac(@secret, salt, @iterations, key_size, "SHA1")
+        end
+      end.new("foo", iterations: 2)
+      caching_generator = ActiveSupport::CachingKeyGenerator.new(key_generator)
+      ActiveSupport::Ractors.make_shareable(caching_generator)
+
+      key = on_ractor(caching_generator) do |caching_generator|
+        caching_generator.generate_key("some_salt", 32)
+      end
+
+      assert_equal key, caching_generator.generate_key("some_salt", 32)
+    end
   end
 end
diff --git a/activesupport/test/ordered_options_test.rb b/activesupport/test/ordered_options_test.rb
index dc41cd8a3c434..b8d7d4323d8c3 100644
--- a/activesupport/test/ordered_options_test.rb
+++ b/activesupport/test/ordered_options_test.rb
@@ -3,8 +3,11 @@
 require "pp"
 require_relative "abstract_unit"
 require "active_support/ordered_options"
+require "active_support/testing/ractors_assertions"
 
 class OrderedOptionsTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::RactorsAssertions
+
   def test_usage
     a = ActiveSupport::OrderedOptions.new
 
@@ -250,6 +253,14 @@ def test_inheritable_options_key
     assert_not object.key?(:four)
   end
 
+  def test_inheritable_options_keys
+    object = ActiveSupport::InheritableOptions.new(one: "first value")
+    object[:two] = "second value"
+    object["three"] = "third value"
+
+    assert_equal [:one, :two, :three], object.keys
+  end
+
   def test_inheritable_options_overridden
     object = ActiveSupport::InheritableOptions.new(one: "first value", two: "second value", three: "third value")
     object["one"] = "first value override"
@@ -347,4 +358,28 @@ def test_inheritable_options_pp
     PP.pp(object, io)
     assert_equal({ one: "first value", two: "second value", three: "third value" }.inspect, io.string.strip)
   end
+
+  def test_inheritable_options_ractor_shareable
+    object = ActiveSupport::InheritableOptions.new(one: "first value")
+    object[:two] = "second value"
+    object["three"] = "third value"
+
+    assert_ractor_make_shareable(object)
+
+    assert_equal "first value", object[:one]
+    assert_equal "second value", object[:two]
+    assert_equal "third value", object[:three]
+  end
+
+  def test_overridden_works_when_frozen
+    object = ActiveSupport::InheritableOptions.new(one: "first value", two: "second value")
+    object[:two] = "second value"
+    object["three"] = "third value"
+
+    object.freeze
+
+    assert object.overridden?(:two)
+    assert_not object.overridden?(:one)
+    assert_not object.overridden?(:three)
+  end
 end
diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index 9f189d54310e1..3e4ab7955d3ea 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -215,9 +215,9 @@ def key_generator(secret_key_base = self.secret_key_base)
     #
     def message_verifiers
       @message_verifiers ||=
-        ActiveSupport::MessageVerifiers.new do |salt, secret_key_base: self.secret_key_base|
-          key_generator(secret_key_base).generate_key(salt)
-        end.rotate_defaults
+        ActiveSupport::MessageVerifiers.new(&ActiveSupport::Ractors.shareable_proc do |salt, secret_key_base: Rails.application.secret_key_base|
+          Rails.application.key_generator(secret_key_base).generate_key(salt)
+        end).rotate_defaults
     end
 
     # Returns a message verifier object.
@@ -668,6 +668,17 @@ def eager_load!
       Rails.autoloaders.each(&:eager_load)
     end
 
+    def ractorize! # :nodoc:
+      warn "WARNING: Rails' Ractor support is experimental. Don't try this at home!", category: :experimental, uplevel: 1
+
+      env_config
+      routes
+
+      @autoloaders, @reloaders, @routes_reloader = nil, nil, nil
+
+      Ractor.make_shareable(self)
+    end
+
   protected
     alias :build_middleware_stack :app
 
@@ -772,7 +783,7 @@ def build_middleware
       end
 
       def coerce_same_site_protection(protection)
-        protection.respond_to?(:call) ? protection : proc { protection }
+        protection.respond_to?(:call) ? protection : ActiveSupport::Ractors.shareable_proc { protection }
       end
 
       def filter_parameters
diff --git a/railties/lib/rails/configuration.rb b/railties/lib/rails/configuration.rb
index 61819fad87a0c..7da834d03983d 100644
--- a/railties/lib/rails/configuration.rb
+++ b/railties/lib/rails/configuration.rb
@@ -116,9 +116,25 @@ def initialize
         @after_generate_callbacks = []
       end
 
+      def freeze
+        return self if self.frozen?
+
+        @aliases.default_proc = nil
+        @options.default_proc = nil
+        @aliases.freeze
+        @options.freeze
+        @fallbacks.freeze
+        @templates.freeze
+        @hidden_namespaces.freeze
+        @after_generate_callbacks.freeze
+        super
+      end
+
       def initialize_copy(source)
         @aliases = @aliases.deep_dup
+        @aliases.default_proc = ->(h, k) { h[k] = {} }
         @options = @options.deep_dup
+        @options.default_proc = ->(h, k) { h[k] = {} }
         @fallbacks = @fallbacks.deep_dup
         @templates = @templates.dup
       end
diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index f726a8c50ea8b..f407a1da688b3 100644
--- a/railties/lib/rails/engine.rb
+++ b/railties/lib/rails/engine.rb
@@ -448,6 +448,12 @@ def initialize
       super
     end
 
+    def freeze
+      app
+      @app_build_lock = nil
+      super
+    end
+
     # Load console and invoke the registered hooks.
     # Check Rails::Railtie.console for more info.
     def load_console(app = self)
diff --git a/railties/lib/rails/initializable.rb b/railties/lib/rails/initializable.rb
index 52a1c841532d0..a52698080c243 100644
--- a/railties/lib/rails/initializable.rb
+++ b/railties/lib/rails/initializable.rb
@@ -47,6 +47,17 @@ def initialize(initializers = nil)
         concat(initializers) if initializers
       end
 
+      def freeze
+        return self if self.frozen?
+
+        @order.default_proc = nil
+        @resolve.default_proc = nil
+        @order.freeze
+        @resolve.freeze
+        @collection.freeze
+        super
+      end
+
       def to_a
         @collection
       end
diff --git a/railties/test/application/ractors_test.rb b/railties/test/application/ractors_test.rb
new file mode 100644
index 0000000000000..072d1a0ffb277
--- /dev/null
+++ b/railties/test/application/ractors_test.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "active_support/testing/ractors_assertions"
+
+if RUBY_VERSION >= "4.0"
+  module ApplicationTests
+    class RactorsTest < ActiveSupport::TestCase
+      include ActiveSupport::Testing::Isolation
+      include ActiveSupport::Testing::RactorsAssertions
+
+      def setup
+        build_app
+
+        add_to_env_config "production", "ActiveSupport::Ractors.unshareable_proc_action = :raise"
+
+        # Remove some defaults that are not compatible
+        add_to_env_config "production", "config.logger = ActiveSupport::Logger.new(nil)"
+        add_to_env_config "production", "config.public_file_server.enabled = false"
+        add_to_env_config "production", "config.cache_store = :null_store"
+        add_to_env_config "production", "config.action_cable.mount_path = nil"
+      end
+
+      def teardown
+        teardown_app
+      end
+
+      test "ractorize! makes the app shareable in production mode" do
+        app "production"
+
+        Rails.application.ractorize!
+
+        assert_ractor_shareable Rails.application
+      end
+    end
+  end
+end
diff --git a/tools/strict_warnings.rb b/tools/strict_warnings.rb
index b20daea67f625..f91baa3dbe125 100644
--- a/tools/strict_warnings.rb
+++ b/tools/strict_warnings.rb
@@ -14,6 +14,8 @@ class WarningError < StandardError; end
     # Expected non-verbose warning emitted by Rails.
     /Ignoring .*\.yml because it has expired/,
     /Failed to validate the schema cache because/,
+    /Rails' Ractor support is experimental/,
+    /Ractor API is experimental and may change in future versions of Ruby/,
   )
 
   SUPPRESSED_WARNINGS = Regexp.union(