diff --git a/actionpack/lib/action_dispatch/http/content_security_policy.rb b/actionpack/lib/action_dispatch/http/content_security_policy.rb
index f544e18e94..9d6aa9bd25 100644
--- a/actionpack/lib/action_dispatch/http/content_security_policy.rb
+++ b/actionpack/lib/action_dispatch/http/content_security_policy.rb
@@ -33,7 +33,11 @@ def initialize(app)
 
       def call(env)
         request = ActionDispatch::Request.new env
-        _, headers, _ = response = @app.call(env)
+        status, headers, _ = response = @app.call(env)
+
+        # Returning CSP headers with a 304 Not Modified is harmful, since nonces in the new
+        # CSP headers might not match nonces in the cached HTML.
+        return response if status == 304
 
         return response if policy_present?(headers)
 
diff --git a/actionpack/test/dispatch/content_security_policy_test.rb b/actionpack/test/dispatch/content_security_policy_test.rb
index 34e75db2ae..5c32f902f3 100644
--- a/actionpack/test/dispatch/content_security_policy_test.rb
+++ b/actionpack/test/dispatch/content_security_policy_test.rb
@@ -440,6 +440,10 @@ def api
       render json: {}
     end
 
+    def not_modified
+      head :not_modified
+    end
+
     private
       def condition?
         params[:condition] == "true"
@@ -457,6 +461,7 @@ def condition?
       get "/style-src", to: "policy#style_src"
       get "/no-policy", to: "policy#no_policy"
       get "/api", to: "policy#api"
+      get "/not-modified", to: "policy#not_modified"
     end
   end
 
@@ -533,6 +538,13 @@ def test_generates_api_security_policy
     assert_policy "default-src 'none'; frame-ancestors 'none'"
   end
 
+  def test_generates_no_content_security_policy_for_not_modified
+    get "/not-modified"
+
+    assert_nil response.headers["Content-Security-Policy"]
+    assert_nil response.headers["Content-Security-Policy-Report-Only"]
+  end
+
   private
     def assert_policy(expected, report_only: false)
       assert_response :success