diff --git a/.changeset/markdown-hr-thickness.md b/.changeset/markdown-hr-thickness.md
new file mode 100644
index 0000000000..2a415fb05b
--- /dev/null
+++ b/.changeset/markdown-hr-thickness.md
@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Set the markdown `<hr>` (thematic break) thickness to 1px using `$border-width`, down from 4px, so it matches Primer's line weight and the ~1px used by other markdown renderers.
diff --git a/src/markdown/markdown-body.scss b/src/markdown/markdown-body.scss
index e2050acc1b..27f1b9861f 100644
--- a/src/markdown/markdown-body.scss
+++ b/src/markdown/markdown-body.scss
@@ -76,7 +76,7 @@
   }
 
   hr {
-    height: $em-spacer-3;
+    height: $border-width;
     padding: 0;
     margin: var(--base-size-24) 0;
     // stylelint-disable-next-line primer/colors