diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 8e169e39..0581c421 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,7 +8,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu ]
-        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0' ]
         include:
           - os: macos
             ruby: '2.7'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d4407e6..ab27cbc9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,11 +15,13 @@ The format is based on [Keep a Changelog], and this project adheres to
 - Improve message when CloudFormation claims there are no changes to apply, even when a template diff is present. ([#398])
 - Display Tags diff (stack tags) in `stack_master diff` and `stack_master apply` commands. ([#397])
 - Resolve style issues identified by RuboCop. ([#396])
+- Test on Ruby 4.0 in the CI build ([#399]).
 
 [Unreleased]: https://github.com/envato/stack_master/compare/v2.17.1...HEAD
 [#396]: https://github.com/envato/stack_master/pull/396
 [#397]: https://github.com/envato/stack_master/pull/397
 [#398]: https://github.com/envato/stack_master/pull/398
+[#399]: https://github.com/envato/stack_master/pull/399
 
 ## [2.17.1] - 2025-12-19