diff --git a/modules/lang/nix/config.el b/modules/lang/nix/config.el
index 3f132ee56..b65f732ad 100644
--- a/modules/lang/nix/config.el
+++ b/modules/lang/nix/config.el
@@ -10,6 +10,14 @@
 (use-package! nix-mode
   :interpreter ("\\(?:cached-\\)?nix-shell" . +nix-shell-init-mode)
   :mode "\\.nix\\'"
+  :init
+  ;; Treat flake.lock files as json. Fall back to js-mode because it's faster
+  ;; than js2-mode, and its extra features aren't needed there.
+  (add-to-list 'auto-mode-alist
+               (cons "/flake\\.lock\\'"
+                     (if (featurep! :lang json)
+                         'json-mode
+                       'js-mode)))
   :config
   (set-repl-handler! 'nix-mode #'+nix/open-repl)
   (set-company-backend! 'nix-mode 'company-nixos-options)