diff --git a/runtime/lua/vim/_core/vimfn.lua b/runtime/lua/vim/_core/vimfn.lua
index e972e7cedb..8cb07d3bbc 100644
--- a/runtime/lua/vim/_core/vimfn.lua
+++ b/runtime/lua/vim/_core/vimfn.lua
@@ -14,7 +14,15 @@ end
 --- Returns all environment variables as a dictionary.
 --- @return table<string, string>
 function M.f_environ()
-  return vim.uv.os_environ()
+  local env = vim.uv.os_environ() ---@type table<string, string>
+  if vim.fn.has('win32') == 1 then -- Vim/legacy behavior: force uppercase keys on Windows. #39443
+    local upper = {} --- @type table<string, string>
+    for k, v in pairs(env) do
+      upper[k:upper()] = v
+    end
+    return upper
+  end
+  return env
 end
 
 return M
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index a04671030a..20147a48cb 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -3386,7 +3386,16 @@ dict_T *create_environment(const dictitem_T *job_env, const bool clear_env, cons
     int envcount;
     if (uv_os_environ(&envitems, &envcount) == 0) {
       for (int i = 0; i < envcount; i++) {
-        tv_dict_add_str(env, envitems[i].name, strlen(envitems[i].name), envitems[i].value);
+        const char *name = envitems[i].name;
+        size_t namelen = strlen(name);
+#ifdef MSWIN
+        // Force uppercase keys on Windows (for dedup, below). #39443
+        if (namelen < MAXPATHL / MB_MAXBYTES) {
+          namelen = mb_strup_buf(name, NameBuff);
+          name = NameBuff;
+        }
+#endif
+        tv_dict_add_str(env, name, namelen, envitems[i].value);
       }
       uv_os_free_environ(envitems, envcount);
     }
@@ -3434,15 +3443,18 @@ dict_T *create_environment(const dictitem_T *job_env, const bool clear_env, cons
   if (job_env) {
 #ifdef MSWIN
     TV_DICT_ITER(job_env->di_tv.vval.v_dict, var, {
-      // Always use upper-case keys for Windows so we detect duplicate keys
-      char *const key = strcase_save(var->di_key, true);
-      size_t len = strlen(key);
-      dictitem_T *dv = tv_dict_find(env, key, len);
+      // Force uppercase keys on Windows (dedup).
+      const char *name = var->di_key;
+      size_t namelen = strlen(name);
+      if (namelen < MAXPATHL / MB_MAXBYTES) {
+        namelen = mb_strup_buf(name, NameBuff);
+        name = NameBuff;
+      }
+      dictitem_T *dv = tv_dict_find(env, name, (ptrdiff_t)namelen);
       if (dv) {
         tv_dict_item_remove(env, dv);
       }
-      tv_dict_add_str(env, key, len, tv_get_string(&var->di_tv));
-      xfree(key);
+      tv_dict_add_str(env, name, namelen, tv_get_string(&var->di_tv));
     });
 #else
     tv_dict_extend(env, job_env->di_tv.vval.v_dict, "force");
diff --git a/src/nvim/strings.c b/src/nvim/strings.c
index c48f971192..22770a9ba5 100644
--- a/src/nvim/strings.c
+++ b/src/nvim/strings.c
@@ -358,6 +358,24 @@ void vim_memcpy_up(char *restrict dst, const char *restrict src, size_t n)
   }
 }
 
+/// Multi-byte uppercase `src` into `dst`. `dst` must have room for the result.
+/// In the worst case, the uppercased form needs `strlen(src) * MB_MAXBYTES` bytes plus NUL.
+///
+/// @return number of bytes written to `dst`, not including the NUL terminator.
+size_t mb_strup_buf(const char *src, char *dst)
+  FUNC_ATTR_NONNULL_ALL
+{
+  size_t i = 0;
+  for (const char *p = src; *p != NUL;) {
+    CharInfo ci = utf_ptr2CharInfo(p);
+    int c = ci.value < 0 ? (uint8_t)(*p) : ci.value;
+    i += (size_t)utf_char2bytes(mb_toupper(c), dst + i);
+    p += ci.len;
+  }
+  dst[i] = NUL;
+  return i;
+}
+
 /// Make given string all upper-case or all lower-case
 ///
 /// Handles multi-byte characters as good as possible.
diff --git a/test/functional/vimscript/environ_spec.lua b/test/functional/vimscript/environ_spec.lua
index 50756f4c8e..30763d3157 100644
--- a/test/functional/vimscript/environ_spec.lua
+++ b/test/functional/vimscript/environ_spec.lua
@@ -27,6 +27,15 @@ describe('vim.fn.environ()', function()
     eq(vim.NIL, n.exec_lua('return vim.env.DOES_NOT_EXIST'))
   end)
 
+  it('forces uppercase keys on Windows #39443', function()
+    if not t.is_os('win') then
+      return
+    end
+    clear({ env = { mixed_Case_Var = 'val' } })
+    eq('val', environ()['MIXED_CASE_VAR'])
+    eq(nil, environ()['mixed_Case_Var'])
+  end)
+
   it('results match getenv()', function()
     clear()
     eq(