diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 4ecba619ba424d7f56463df3789363edb6a39167..9daeaff8ab9b5133ece1427159323d038d98ef35 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -26,6 +26,7 @@
     which provides a mechanism for custom type errors that reports the errors in
     a more predictable behaviour than `TypeError`.
   * Make `(&)` representation polymorphic in the return type ([CLC proposal #158](https://github.com/haskell/core-libraries-committee/issues/158))
+  * Implement `GHC.IORef.atomicSwapIORef` via a new dedicated primop `atomicSwapMutVar#` ([CLC proposal #139](https://github.com/haskell/core-libraries-committee/issues/139))
 
 ## 4.18.0.0 *March 2023*
   * Shipped with GHC 9.6.1