diff --git a/.gitignore b/.gitignore
index 2b16924f1047..db9665009993 100644
--- a/.gitignore
+++ b/.gitignore
@@ -181,3 +181,6 @@ testing/raptor/webext/raptor/auto_gen_test_config.js
 # Ignore ICU4X experimentation data files.
 # See intl/ICU4X.md for more details.
 config/external/icu4x
+
+# Ignore the index files generated by clangd.
+.cache/clangd/index/
diff --git a/.hgignore b/.hgignore
index 5e294df5fc7b..428df0dff0a5 100644
--- a/.hgignore
+++ b/.hgignore
@@ -228,3 +228,6 @@ toolkit/components/certviewer/content/package-lock.json
 # Ignore ICU4X experimentation data files.
 # See intl/ICU4X.md for more details.
 ^config/external/icu4x
+
+# Ignore the index files generated by clangd.
+^.cache/clangd/index/