diff --git a/news/changelog-1.9.md b/news/changelog-1.9.md
index c806b09e320..f5b92e4d39b 100644
--- a/news/changelog-1.9.md
+++ b/news/changelog-1.9.md
@@ -11,6 +11,10 @@ All changes included in 1.9:
 
 ## Formats
 
+### `gfm`
+
+- ([#13421](https://github.com/quarto-dev/quarto-cli/issues/13421)): Do not word-wrap titles in header.
+
 ### `html`
 
 - ([#13413](https://github.com/quarto-dev/quarto-cli/issues/13413)): Fix uncentered play button in `video` shortcodes from cross-reference divs. (author: @bruvellu)
diff --git a/src/resources/pandoc/templates/default.markdown b/src/resources/pandoc/templates/default.markdown
index 009535a22ec..7efb487c160 100644
--- a/src/resources/pandoc/templates/default.markdown
+++ b/src/resources/pandoc/templates/default.markdown
@@ -1,5 +1,5 @@
 $if(title)$
-# $title$
+# $title/nowrap$
 $endif$
 $for(by-author)$$it.name.literal$$sep$, $endfor$
 $if(date)$
diff --git a/tests/docs/smoke-all/2025/09/26/issue-13421.qmd b/tests/docs/smoke-all/2025/09/26/issue-13421.qmd
new file mode 100644
index 00000000000..7bf4443267e
--- /dev/null
+++ b/tests/docs/smoke-all/2025/09/26/issue-13421.qmd
@@ -0,0 +1,12 @@
+---
+format: gfm
+title: "0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789"
+_quarto:
+  tests:
+    gfm:
+      ensureFileRegexMatches:
+        - []
+        - ["\n012345"]
+---
+
+Do not wrap.
\ No newline at end of file