diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index e2e6aba17e..3ceefb8a1f 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -9,7 +9,7 @@ git-merge - Join two or more development histories together
 SYNOPSIS
 --------
 [verse]
-'git merge' [-n] [--stat] [--no-commit] [--squash]
+'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
 	[-s <strategy>] [-X <strategy-option>]
 	[--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
 'git merge' <msg> HEAD <commit>...
diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index 6bd0b041c3..f2f1d0f51c 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -8,10 +8,20 @@ failed and do not autocommit, to give the user a chance to
 inspect and further tweak the merge result before committing.
 
 --edit::
--e::
+--no-edit::
+	Invoke an editor before committing successful mechanical merge to
+	further edit the auto-generated merge message, so that the user
+	can explain and justify the merge. The `--no-edit` option can be
+	used to accept the auto-generated message (this is generally
+	discouraged). The `--edit` option is still useful if you are
+	giving a draft message with the `-m` option from the command line
+	and want to edit it in the editor.
 +
-	Invoke editor before committing successful merge to further
-	edit the default merge message.
+Older scripts may depend on the historical behaviour of not allowing the
+user to edit the merge log message. They will see an editor opened when
+they run `git merge`. To make it easier to adjust such scripts to the
+updated behaviour, the environment variable `GIT_MERGE_AUTOEDIT` can be
+set to `no` at the beginning of them.
 
 --ff::
 --no-ff::
diff --git a/builtin/merge.c b/builtin/merge.c
index 99f1429b35..0006175d15 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -46,7 +46,7 @@ static const char * const builtin_merge_usage[] = {
 
 static int show_diffstat = 1, shortlog_len, squash;
 static int option_commit = 1, allow_fast_forward = 1;
-static int fast_forward_only, option_edit;
+static int fast_forward_only, option_edit = -1;
 static int allow_trivial = 1, have_message;
 static struct strbuf merge_msg;
 static struct commit_list *remoteheads;
@@ -189,7 +189,7 @@ static struct option builtin_merge_options[] = {
 		"create a single commit instead of doing a merge"),
 	OPT_BOOLEAN(0, "commit", &option_commit,
 		"perform a commit if the merge succeeds (default)"),
-	OPT_BOOLEAN('e', "edit", &option_edit,
+	OPT_BOOL('e', "edit", &option_edit,
 		"edit message before committing"),
 	OPT_BOOLEAN(0, "ff", &allow_fast_forward,
 		"allow fast-forward (default)"),
@@ -877,12 +877,12 @@ static void prepare_to_commit(void)
 	write_merge_msg(&msg);
 	run_hook(get_index_file(), "prepare-commit-msg",
 		 git_path("MERGE_MSG"), "merge", NULL, NULL);
-	if (option_edit) {
+	if (0 < option_edit) {
 		if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
 			abort_commit(NULL);
 	}
 	read_merge_msg(&msg);
-	stripspace(&msg, option_edit);
+	stripspace(&msg, 0 < option_edit);
 	if (!msg.len)
 		abort_commit(_("Empty commit message."));
 	strbuf_release(&merge_msg);
@@ -1076,6 +1076,33 @@ static void write_merge_state(void)
 	close(fd);
 }
 
+static int default_edit_option(void)
+{
+	static const char name[] = "GIT_MERGE_AUTOEDIT";
+	const char *e = getenv(name);
+	struct stat st_stdin, st_stdout;
+
+	if (have_message)
+		/* an explicit -m msg without --[no-]edit */
+		return 0;
+
+	if (e) {
+		int v = git_config_maybe_bool(name, e);
+		if (v < 0)
+			die("Bad value '%s' in environment '%s'", e, name);
+		return v;
+	}
+
+	/* Use editor if stdin and stdout are the same and is a tty */
+	return (!fstat(0, &st_stdin) &&
+		!fstat(1, &st_stdout) &&
+		isatty(0) &&
+		st_stdin.st_dev == st_stdout.st_dev &&
+		st_stdin.st_ino == st_stdout.st_ino &&
+		st_stdin.st_mode == st_stdout.st_mode);
+}
+
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	unsigned char result_tree[20];
@@ -1261,6 +1288,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (option_edit < 0)
+		option_edit = default_edit_option();
+
 	if (!use_strategies) {
 		if (!remoteheads->next)
 			add_strategies(pull_twohead, DEFAULT_TWOHEAD);
diff --git a/t/test-lib.sh b/t/test-lib.sh
index bdd9513b84..ed32c2ab7b 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -63,7 +63,8 @@ GIT_AUTHOR_NAME='A U Thor'
 GIT_COMMITTER_EMAIL=committer@example.com
 GIT_COMMITTER_NAME='C O Mitter'
 GIT_MERGE_VERBOSITY=5
-export GIT_MERGE_VERBOSITY
+GIT_MERGE_AUTOEDIT=no
+export GIT_MERGE_VERBOSITY GIT_MERGE_AUTOEDIT
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR