diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 58731c19422..569485815a0 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -3,26 +3,73 @@ git-merge-tree(1)
 
 NAME
 ----
-git-merge-tree - Show three-way merge without touching index
+git-merge-tree - Perform merge without touching index or working tree
 
 
 SYNOPSIS
 --------
 [verse]
-'git merge-tree' <base-tree> <branch1> <branch2>
+'git merge-tree' [--write-tree] <branch1> <branch2>
+'git merge-tree' [--trivial-merge] <base-tree> <branch1> <branch2> (deprecated)
 
 DESCRIPTION
 -----------
-Reads three tree-ish, and output trivial merge results and
-conflicting stages to the standard output.  This is similar to
-what three-way 'git read-tree -m' does, but instead of storing the
-results in the index, the command outputs the entries to the
-standard output.
-
-This is meant to be used by higher level scripts to compute
-merge results outside of the index, and stuff the results back into the
-index.  For this reason, the output from the command omits
-entries that match the <branch1> tree.
+
+Performs a merge, but does not make any new commits and does not read
+from or write to either the working tree or index.
+
+The second form is deprecated and supported only for backward
+compatibility.  It will likely be removed in the future, and will not
+be discussed further in this manual.
+
+The first form will merge the two branches, doing a real merge.  A real
+merge is distinguished from a trivial merge in that it includes:
+
+  * three way content merges of individual files
+  * rename detection
+  * proper directory/file conflict handling
+  * recursive ancestor consolidation (i.e. when there is more than one
+    merge base, creating a virtual merge base by merging the merge bases)
+  * etc.
+
+After the merge completes, it will create a new toplevel tree object.
+See `OUTPUT` below for details.
+
+OUTPUT
+------
+
+For either a successful or conflicted merge, the output from
+git-merge-tree is simply one line:
+
+	<OID of toplevel tree>
+
+The printed tree object corresponds to what would be checked out in
+the working tree at the end of `git merge`, and thus may have files
+with conflict markers in them.
+
+EXIT STATUS
+-----------
+
+For a successful, non-conflicted merge, the exit status is 0.  When the
+merge has conflicts, the exit status is 1.  If the merge is not able to
+complete (or start) due to some kind of error, the exit status is
+something other than 0 or 1.
+
+USAGE NOTES
+-----------
+
+git-merge-tree was written to be low-level plumbing, similar to
+hash-object, mktree, commit-tree, update-ref, and mktag.  Thus, it could
+be used as a part of a series of steps such as
+
+       NEWTREE=$(git merge-tree --write-tree $BRANCH1 $BRANCH2)
+       test $? -eq 0 || die "There were conflicts..."
+       NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 -p $BRANCH2)
+       git update-ref $BRANCH1 $NEWCOMMIT
+
+However, it does not quite fit into the same category of low-level
+plumbing commands since the possibility of merge conflicts give it a
+much higher chance of the command not succeeding.
 
 GIT
 ---
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index e98ec8a9f1d..d14c9f6e44e 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -2,6 +2,9 @@
 #include "builtin.h"
 #include "tree-walk.h"
 #include "xdiff-interface.h"
+#include "help.h"
+#include "commit-reach.h"
+#include "merge-ort.h"
 #include "object-store.h"
 #include "parse-options.h"
 #include "repository.h"
@@ -392,7 +395,46 @@ struct merge_tree_options {
 static int real_merge(struct merge_tree_options *o,
 		      const char *branch1, const char *branch2)
 {
-	die(_("real merges are not yet implemented"));
+	struct commit *parent1, *parent2;
+	struct commit_list *common;
+	struct commit_list *merge_bases = NULL;
+	struct commit_list *j;
+	struct merge_options opt;
+	struct merge_result result = { 0 };
+
+	parent1 = get_merge_parent(branch1);
+	if (!parent1)
+		help_unknown_ref(branch1, "merge-tree",
+				 _("not something we can merge"));
+
+	parent2 = get_merge_parent(branch2);
+	if (!parent2)
+		help_unknown_ref(branch2, "merge-tree",
+				 _("not something we can merge"));
+
+	init_merge_options(&opt, the_repository);
+
+	opt.show_rename_progress = 0;
+
+	opt.branch1 = branch1;
+	opt.branch2 = branch2;
+
+	/*
+	 * Get the merge bases, in reverse order; see comment above
+	 * merge_incore_recursive in merge-ort.h
+	 */
+	common = get_merge_bases(parent1, parent2);
+	if (!common)
+		die(_("refusing to merge unrelated histories"));
+	for (j = common; j; j = j->next)
+		commit_list_insert(j->item, &merge_bases);
+
+	merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
+	if (result.clean < 0)
+		die(_("failure to merge"));
+	puts(oid_to_hex(&result.tree->object.oid));
+	merge_finalize(&opt, &result);
+	return !result.clean; /* result.clean < 0 handled above */
 }
 
 int cmd_merge_tree(int argc, const char **argv, const char *prefix)
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
new file mode 100755
index 00000000000..66c3eaf2021
--- /dev/null
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -0,0 +1,88 @@
+#!/bin/sh
+
+test_description='git merge-tree --write-tree'
+
+. ./test-lib.sh
+
+# This test is ort-specific
+if test "${GIT_TEST_MERGE_ALGORITHM}" != "ort"
+then
+	skip_all="GIT_TEST_MERGE_ALGORITHM != ort"
+	test_done
+fi
+
+test_expect_success setup '
+	test_write_lines 1 2 3 4 5 >numbers &&
+	echo hello >greeting &&
+	echo foo >whatever &&
+	git add numbers greeting whatever &&
+	test_tick &&
+	git commit -m initial &&
+
+	git branch side1 &&
+	git branch side2 &&
+
+	git checkout side1 &&
+	test_write_lines 1 2 3 4 5 6 >numbers &&
+	echo hi >greeting &&
+	echo bar >whatever &&
+	git add numbers greeting whatever &&
+	test_tick &&
+	git commit -m modify-stuff &&
+
+	git checkout side2 &&
+	test_write_lines 0 1 2 3 4 5 >numbers &&
+	echo yo >greeting &&
+	git rm whatever &&
+	mkdir whatever &&
+	>whatever/empty &&
+	git add numbers greeting whatever/empty &&
+	test_tick &&
+	git commit -m other-modifications
+'
+
+test_expect_success 'Content merge and a few conflicts' '
+	git checkout side1^0 &&
+	test_must_fail git merge side2 &&
+	expected_tree=$(cat .git/AUTO_MERGE) &&
+
+	# We will redo the merge, while we are still in a conflicted state!
+	test_when_finished "git reset --hard" &&
+
+	test_expect_code 1 git merge-tree --write-tree side1 side2 >RESULT &&
+	actual_tree=$(head -n 1 RESULT) &&
+
+	# Due to differences of e.g. "HEAD" vs "side1", the results will not
+	# exactly match.  Dig into individual files.
+
+	# Numbers should have three-way merged cleanly
+	test_write_lines 0 1 2 3 4 5 6 >expect &&
+	git show ${actual_tree}:numbers >actual &&
+	test_cmp expect actual &&
+
+	# whatever and whatever~<branch> should have same HASHES
+	git rev-parse ${expected_tree}:whatever ${expected_tree}:whatever~HEAD >expect &&
+	git rev-parse ${actual_tree}:whatever ${actual_tree}:whatever~side1 >actual &&
+	test_cmp expect actual &&
+
+	# greeting should have a merge conflict
+	git show ${expected_tree}:greeting >tmp &&
+	cat tmp | sed -e s/HEAD/side1/ >expect &&
+	git show ${actual_tree}:greeting >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Barf on misspelled option, with exit code other than 0 or 1' '
+	# Mis-spell with single "s" instead of double "s"
+	test_expect_code 129 git merge-tree --write-tree --mesages FOOBAR side1 side2 2>expect &&
+
+	grep "error: unknown option.*mesages" expect
+'
+
+test_expect_success 'Barf on too many arguments' '
+	test_expect_code 129 git merge-tree --write-tree side1 side2 side3 2>expect &&
+
+	grep "^usage: git merge-tree" expect
+'
+
+test_done
