diff --git a/actionpack/lib/action_dispatch/middleware/cookies.rb b/actionpack/lib/action_dispatch/middleware/cookies.rb
index 3865557ad9..ac58447233 100644
--- a/actionpack/lib/action_dispatch/middleware/cookies.rb
+++ b/actionpack/lib/action_dispatch/middleware/cookies.rb
@@ -70,7 +70,7 @@ module ActionDispatch
     end
 
     def cookies_same_site_protection
-      get_header Cookies::COOKIES_SAME_SITE_PROTECTION
+      get_header(Cookies::COOKIES_SAME_SITE_PROTECTION) || Proc.new { }
     end
 
     def cookies_digest
@@ -444,7 +444,9 @@ module ActionDispatch
           end
 
           options[:path]      ||= "/"
-          options[:same_site] ||= request.cookies_same_site_protection
+
+          cookies_same_site_protection = request.cookies_same_site_protection
+          options[:same_site] ||= cookies_same_site_protection.call(request)
 
           if options[:domain] == :all || options[:domain] == "all"
             # If there is a provided tld length then we use it otherwise default domain regexp.
diff --git a/actionpack/test/dispatch/cookies_test.rb b/actionpack/test/dispatch/cookies_test.rb
index a6291422b0..23716c0aeb 100644
--- a/actionpack/test/dispatch/cookies_test.rb
+++ b/actionpack/test/dispatch/cookies_test.rb
@@ -358,20 +358,41 @@ class CookiesTest < ActionController::TestCase
     @request.env["action_dispatch.encrypted_signed_cookie_salt"] = ENCRYPTED_SIGNED_COOKIE_SALT
     @request.env["action_dispatch.authenticated_encrypted_cookie_salt"] = AUTHENTICATED_ENCRYPTED_COOKIE_SALT
 
-    @request.env["action_dispatch.cookies_same_site_protection"] = :lax
+    @request.env["action_dispatch.cookies_same_site_protection"] = proc { :lax }
     @request.host = "www.nextangle.com"
   end
 
   def test_setting_cookie_with_no_protection
-    @request.env["action_dispatch.cookies_same_site_protection"] = :none
+    @request.env["action_dispatch.cookies_same_site_protection"] = proc { :none }
 
     get :authenticate
     assert_cookie_header "user_name=david; path=/; SameSite=None"
     assert_equal({ "user_name" => "david" }, @response.cookies)
   end
 
+  def test_setting_cookie_with_protection_proc_normal_user_agent
+    @request.env["action_dispatch.cookies_same_site_protection"] = Proc.new do |request|
+      :strict unless request.user_agent == "spooky browser"
+    end
+
+    get :authenticate
+    assert_cookie_header "user_name=david; path=/; SameSite=Strict"
+    assert_equal({ "user_name" => "david" }, @response.cookies)
+  end
+
+  def test_setting_cookie_with_protection_proc_special_user_agent
+    @request.env["action_dispatch.cookies_same_site_protection"] = Proc.new do |request|
+      :strict unless request.user_agent == "spooky browser"
+    end
+
+    request.user_agent = "spooky browser"
+    get :authenticate
+    assert_cookie_header "user_name=david; path=/"
+    assert_equal({ "user_name" => "david" }, @response.cookies)
+  end
+
   def test_setting_cookie_with_misspelled_protection_raises
-    @request.env["action_dispatch.cookies_same_site_protection"] = :funky
+    @request.env["action_dispatch.cookies_same_site_protection"] = proc { :funky }
 
     error = assert_raise ArgumentError do
       get :authenticate
@@ -380,7 +401,7 @@ class CookiesTest < ActionController::TestCase
   end
 
   def test_setting_cookie_with_strict
-    @request.env["action_dispatch.cookies_same_site_protection"] = :strict
+    @request.env["action_dispatch.cookies_same_site_protection"] = proc { :strict }
 
     get :authenticate
     assert_cookie_header "user_name=david; path=/; SameSite=Strict"
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index cd43e9bd91..ebaf7476c1 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -629,7 +629,15 @@ Defaults to `'signed cookie'`.
 
 * `config.action_dispatch.cookies_same_site_protection` configures the default
   value of the `SameSite` attribute when setting cookies. When set to `nil`, the
-  `SameSite` attribute is not added.
+  `SameSite` attribute is not added. To allow the value of the `SameSite` attribute
+  to be configured dynamically based on the request, a proc may be specified.
+  For example:
+
+    ```ruby
+    config.action_dispatch.cookies_same_site_protection = ->(request) do
+      :strict unless request.user_agent == "TestAgent"
+    end
+    ```
 
 * `config.action_dispatch.ssl_default_redirect_status` configures the default
   HTTP status code used when redirecting non-GET/HEAD requests from HTTP to HTTPS
diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index 864fd63de9..ed0798e295 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -281,7 +281,7 @@ module Rails
           "action_dispatch.cookies_serializer" => config.action_dispatch.cookies_serializer,
           "action_dispatch.cookies_digest" => config.action_dispatch.cookies_digest,
           "action_dispatch.cookies_rotations" => config.action_dispatch.cookies_rotations,
-          "action_dispatch.cookies_same_site_protection" => config.action_dispatch.cookies_same_site_protection,
+          "action_dispatch.cookies_same_site_protection" => coerce_same_site_protection(config.action_dispatch.cookies_same_site_protection),
           "action_dispatch.use_cookies_with_metadata" => config.action_dispatch.use_cookies_with_metadata,
           "action_dispatch.content_security_policy" => config.content_security_policy,
           "action_dispatch.content_security_policy_report_only" => config.content_security_policy_report_only,
@@ -628,5 +628,9 @@ module Rails
       def build_middleware
         config.app_middleware + super
       end
+
+      def coerce_same_site_protection(protection)
+        protection.respond_to?(:call) ? protection : proc { protection }
+      end
   end
 end