diff --git a/lib/linguist/languages.yml b/lib/linguist/languages.yml
index 42ce978d..aff5e8b0 100644
--- a/lib/linguist/languages.yml
+++ b/lib/linguist/languages.yml
@@ -987,6 +987,12 @@ IRC log:
   - .irclog
   - .weechatlog
 
+Isabelle:
+  type: programming
+  color: "#fdcd00"
+  extensions:
+  - .thy
+
 Io:
   type: programming
   color: "#a9188d"