diff --git a/CHANGES.md b/CHANGES.md
index 81dc16cae5b..aaf56c5e8d8 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,10 @@
+Unreleased
+----------
+
+- Fixed a bug that could result in needless recompilation under Windows due to
+  case differences in the result of `Sys.getcwd` (observed under `emacs`).
+  (#3966, @nojb).
+
 2.8.2 (21/01/2021)
 ------------------
 
diff --git a/bin/common.ml b/bin/common.ml
index cac11b66a7c..9b0fa2da61f 100644
--- a/bin/common.ml
+++ b/bin/common.ml
@@ -86,9 +86,18 @@ let prefix_target common s = common.target_prefix ^ s
 
 let instrument_with t = t.instrument_with
 
+(* To avoid needless recompilations under Windows, where the case of
+   [Sys.getcwd] can vary between different invocations of [dune], normalize to
+   lowercase. *)
+let normalize_path p =
+  if Sys.win32 then
+    Path.External.lowercase_ascii p
+  else
+    p
+
 let set_dirs c =
   if c.root.dir <> Filename.current_dir_name then Sys.chdir c.root.dir;
-  Path.set_root (Path.External.cwd ());
+  Path.set_root (normalize_path (Path.External.cwd ()));
   Path.Build.set_build_dir (Path.Build.Kind.of_string c.build_dir)
 
 let set_common_other ?log_file c ~targets =
diff --git a/src/stdune/path.ml b/src/stdune/path.ml
index fb10e1add8d..af3928e0fe3 100644
--- a/src/stdune/path.ml
+++ b/src/stdune/path.ml
@@ -50,6 +50,8 @@ module External : sig
   val cwd : unit -> t
 
   val as_local : t -> string
+
+  val lowercase_ascii : t -> t
 end = struct
   module T =
     Interned.No_interning
@@ -164,6 +166,8 @@ end = struct
     else
       String.is_prefix ~prefix:(to_string a ^ "/") (to_string b)
 
+  let lowercase_ascii t = make (String.lowercase_ascii (to_string t))
+
   module Set = struct
     include T.Set
 
diff --git a/src/stdune/path.mli b/src/stdune/path.mli
index 07b8b06cf2b..e171b14ef8e 100644
--- a/src/stdune/path.mli
+++ b/src/stdune/path.mli
@@ -111,6 +111,8 @@ module External : sig
   val relative : t -> string -> t
 
   val mkdir_p : ?perms:int -> t -> unit
+
+  val lowercase_ascii : t -> t
 end
 
 module Build : sig