diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1043,7 +1043,7 @@
 coreconfigitem(
     b'experimental',
     b'mergetempdirprefix',
-    default=None,
+    default=b'hgmerge',
 )
 coreconfigitem(
     b'experimental',
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -926,22 +926,16 @@
     copies `localpath` to another temporary file, so an external merge tool may
     use them.
     """
-    tmproot = None
     tmprootprefix = repo.ui.config(b'experimental', b'mergetempdirprefix')
-    if tmprootprefix:
-        tmproot = pycompat.mkdtemp(prefix=tmprootprefix)
+    tmproot = pycompat.mkdtemp(prefix=tmprootprefix)
 
     def maketempfrompath(prefix, path):
         fullbase, ext = os.path.splitext(path)
         pre = b"%s~%s" % (os.path.basename(fullbase), prefix)
-        if tmproot:
-            name = os.path.join(tmproot, pre)
-            if ext:
-                name += ext
-            f = open(name, "wb")
-        else:
-            fd, name = pycompat.mkstemp(prefix=pre + b'.', suffix=ext)
-            f = os.fdopen(fd, "wb")
+        name = os.path.join(tmproot, pre)
+        if ext:
+            name += ext
+        f = open(name, "wb")
         return f, name
 
     def tempfromcontext(prefix, ctx):
diff --git a/tests/test-merge-tools.t b/tests/test-merge-tools.t
--- a/tests/test-merge-tools.t
+++ b/tests/test-merge-tools.t
@@ -1592,7 +1592,7 @@
   arg: "ll:working copy"
   arg: "lo:"
   arg: "merge rev"
-  arg: "lb:common ancestor: */f~base.*" (glob)
+  arg: "lb:common ancestor: */f~base" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1608,7 +1608,7 @@
   $ cat <<EOF > printargs_merge_tool
   > while test \$# -gt 0; do echo arg: \""\$1"\"; shift; done
   > EOF
-  $ hg --config experimental.mergetempdirprefix=$TESTTMP/hgmerge. \
+  $ hg --config experimental.mergetempdirprefix=$TESTTMP/custom. \
   >    --config merge-tools.true.executable='sh' \
   >    --config merge-tools.true.args='./printargs_merge_tool ll:$labellocal lo: $labelother lb:$labelbase": "$base' \
   >    --config merge-tools.true.mergemarkertemplate='tooltmpl {short(node)}' \
@@ -1619,7 +1619,7 @@
   arg: "ll:working copy"
   arg: "lo:"
   arg: "merge rev"
-  arg: "lb:common ancestor: */hgmerge.*/f~base" (glob)
+  arg: "lb:common ancestor: */custom.*/f~base" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1649,7 +1649,7 @@
   arg: "ll:working copy: tooltmpl ef83787e2614"
   arg: "lo:"
   arg: "merge rev: tooltmpl 0185f4e0cf02"
-  arg: "lb:common ancestor: */f~base.*" (glob)
+  arg: "lb:common ancestor: */f~base" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1895,7 +1895,7 @@
   $ hg update -q -C 2
   $ hg merge -y -r tip --tool echo --config merge-tools.echo.args='$base $local $other $output'
   merging f and f.txt to f.txt
-  */f~base.* */f~local.*.txt */f~other.*.txt $TESTTMP/repo/f.txt (glob)
+  */f~base */f~local.txt */f~other.txt $TESTTMP/repo/f.txt (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
@@ -1909,9 +1909,9 @@
   $ hg update -q -C 2
   $ hg merge -y -r tip --tool echo \
   >    --config merge-tools.echo.args='$base $local $other $output' \
-  >    --config experimental.mergetempdirprefix=$TESTTMP/hgmerge.
+  >    --config experimental.mergetempdirprefix=$TESTTMP/custom.
   merging f and f.txt to f.txt
-  $TESTTMP/hgmerge.*/f~base $TESTTMP/hgmerge.*/f~local.txt $TESTTMP/hgmerge.*/f~other.txt $TESTTMP/repo/f.txt (glob)
+  $TESTTMP/custom.*/f~base $TESTTMP/custom.*/f~local.txt $TESTTMP/custom.*/f~other.txt $TESTTMP/repo/f.txt (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)