diff --git a/UNRELEASED-v9.md b/UNRELEASED-v9.md
index f9da39e930d..7565447f00e 100644
--- a/UNRELEASED-v9.md
+++ b/UNRELEASED-v9.md
@@ -19,6 +19,7 @@ Use [the changelog guidelines](/documentation/Versioning%20and%20changelog.md) t
 
 **Sass functions and mixins**
 
+- Removed the `border-width()` function ([#4900](https://github.com/Shopify/polaris-react/pull/4900))
 - Removed the `filter()` function ([#4676](https://github.com/Shopify/polaris-react/pull/4676))
 - Removed the `px()` function ([#4751](https://github.com/Shopify/polaris-react/pull/4751))
 - Removed the `em()` function ([#4937](https://github.com/Shopify/polaris-react/pull/4937))
diff --git a/documentation/guides/migrating-from-v8-to-v9.md b/documentation/guides/migrating-from-v8-to-v9.md
index 5cb918a6576..a11b2a4c54e 100644
--- a/documentation/guides/migrating-from-v8-to-v9.md
+++ b/documentation/guides/migrating-from-v8-to-v9.md
@@ -123,6 +123,12 @@ A list of functions/mixins and their value equivalents or new token values.
 
 #### `border-width()`
 
+| Function                                 | Replacement Value/Token |
+| ---------------------------------------- | ----------------------- |
+| `border-width()`<br>`border-width(base)` | `--p-border-width-1`    |
+| `border-width(thick)`                    | `--p-border-width-2`    |
+| `border-width(thicker)`                  | `--p-border-width-3`    |
+
 #### `duration()`
 
 | Function                         | Replacement Value/Token |