diff --git a/builtin/commit-tree.c b/builtin/commit-tree.c
index b9c3312..d5e19af 100644
--- a/builtin/commit-tree.c
+++ b/builtin/commit-tree.c
@@ -8,8 +8,9 @@
 #include "tree.h"
 #include "builtin.h"
 #include "utf8.h"
+#include "gpg-interface.h"
 
-static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-m <message>] [-F <file>] <sha1> <changelog";
+static const char commit_tree_usage[] = "git commit-tree [(-p <sha1>)...] [-S<signer>] [-m <message>] [-F <file>] <sha1> <changelog";
 
 static void new_parent(struct commit *parent, struct commit_list **parents_p)
 {
@@ -25,6 +26,14 @@
 	commit_list_insert(parent, parents_p);
 }
 
+static int commit_tree_config(const char *var, const char *value, void *cb)
+{
+	int status = git_gpg_config(var, value, NULL);
+	if (status)
+		return status;
+	return git_default_config(var, value, cb);
+}
+
 int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 {
 	int i, got_tree = 0;
@@ -32,12 +41,16 @@
 	unsigned char tree_sha1[20];
 	unsigned char commit_sha1[20];
 	struct strbuf buffer = STRBUF_INIT;
+	const char *sign_commit = NULL;
 
-	git_config(git_default_config, NULL);
+	git_config(commit_tree_config, NULL);
 
 	if (argc < 2 || !strcmp(argv[1], "-h"))
 		usage(commit_tree_usage);
 
+	if (get_sha1(argv[1], tree_sha1))
+		die("Not a valid object name %s", argv[1]);
+
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
 		if (!strcmp(arg, "-p")) {
@@ -51,6 +64,11 @@
 			continue;
 		}
 
+		if (!memcmp(arg, "-S", 2)) {
+			sign_commit = arg + 2;
+			continue;
+		}
+
 		if (!strcmp(arg, "-m")) {
 			if (argc <= ++i)
 				usage(commit_tree_usage);
@@ -98,7 +116,8 @@
 			die_errno("git commit-tree: failed to read");
 	}
 
-	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1, NULL)) {
+	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1,
+			NULL, sign_commit)) {
 		strbuf_release(&buffer);
 		return 1;
 	}
diff --git a/builtin/commit.c b/builtin/commit.c
index fca7ea0..7e8a1cf 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -26,6 +26,7 @@
 #include "unpack-trees.h"
 #include "quote.h"
 #include "submodule.h"
+#include "gpg-interface.h"
 
 static const char * const builtin_commit_usage[] = {
 	"git commit [options] [--] <filepattern>...",
@@ -85,6 +86,8 @@
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static int no_post_rewrite, allow_empty_message;
 static char *untracked_files_arg, *force_date, *ignore_submodule_arg;
+static char *sign_commit;
+
 /*
  * The default commit message cleanup mode will remove the lines
  * beginning with # (shell comments) and leading and trailing
@@ -144,6 +147,8 @@
 	OPT_BOOLEAN('e', "edit", &edit_flag, "force edit of commit"),
 	OPT_STRING(0, "cleanup", &cleanup_arg, "default", "how to strip spaces and #comments from message"),
 	OPT_BOOLEAN(0, "status", &include_status, "include status in commit message template"),
+	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, "key id",
+	  "GPG sign commit", PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 	/* end commit message options */
 
 	OPT_GROUP("Commit contents options"),
@@ -1324,6 +1329,7 @@
 static int git_commit_config(const char *k, const char *v, void *cb)
 {
 	struct wt_status *s = cb;
+	int status;
 
 	if (!strcmp(k, "commit.template"))
 		return git_config_pathname(&template_file, k, v);
@@ -1332,6 +1338,9 @@
 		return 0;
 	}
 
+	status = git_gpg_config(k, v, NULL);
+	if (status)
+		return status;
 	return git_status_config(k, v, s);
 }
 
@@ -1488,7 +1497,7 @@
 		extra = read_commit_extra_headers(current_head);
 
 	if (commit_tree_extended(sb.buf, active_cache_tree->sha1, parents, sha1,
-				 author_ident.buf, extra)) {
+				 author_ident.buf, sign_commit, extra)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));
 	}
diff --git a/builtin/merge.c b/builtin/merge.c
index 99f1429..e5afe64 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -26,6 +26,7 @@
 #include "merge-recursive.h"
 #include "resolve-undo.h"
 #include "remote.h"
+#include "gpg-interface.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -62,6 +63,7 @@
 static int abort_current_merge;
 static int show_progress = -1;
 static int default_to_upstream;
+static const char *sign_commit;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -207,6 +209,8 @@
 	OPT_BOOLEAN(0, "abort", &abort_current_merge,
 		"abort the current in-progress merge"),
 	OPT_SET_INT(0, "progress", &show_progress, "force progress reporting", 1),
+	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, "key id",
+	  "GPG sign commit", PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 	OPT_END()
 };
 
@@ -533,6 +537,8 @@
 
 static int git_merge_config(const char *k, const char *v, void *cb)
 {
+	int status;
+
 	if (branch && !prefixcmp(k, "branch.") &&
 		!prefixcmp(k + 7, branch) &&
 		!strcmp(k + 7 + strlen(branch), ".mergeoptions")) {
@@ -570,6 +576,10 @@
 		default_to_upstream = git_config_bool(k, v);
 		return 0;
 	}
+
+	status = git_gpg_config(k, v, NULL);
+	if (status)
+		return status;
 	return git_diff_ui_config(k, v, cb);
 }
 
@@ -902,7 +912,8 @@
 	parent->next->item = remoteheads->item;
 	parent->next->next = NULL;
 	prepare_to_commit();
-	commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL);
+	commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL,
+		    sign_commit);
 	finish(head, result_commit, "In-index merge");
 	drop_save();
 	return 0;
@@ -933,7 +944,8 @@
 	strbuf_addch(&merge_msg, '\n');
 	prepare_to_commit();
 	free_commit_list(remoteheads);
-	commit_tree(merge_msg.buf, result_tree, parents, result_commit, NULL);
+	commit_tree(merge_msg.buf, result_tree, parents, result_commit,
+		    NULL, sign_commit);
 	strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
 	finish(head, result_commit, buf.buf);
 	strbuf_release(&buf);
diff --git a/commit.c b/commit.c
index b781274..f00076e 100644
--- a/commit.c
+++ b/commit.c
@@ -6,6 +6,7 @@
 #include "diff.h"
 #include "revision.h"
 #include "notes.h"
+#include "gpg-interface.h"
 
 int save_commit_buffer = 1;
 
@@ -840,6 +841,42 @@
 	return result;
 }
 
+static const char gpg_sig_header[] = "gpgsig";
+static const int gpg_sig_header_len = sizeof(gpg_sig_header) - 1;
+
+static int do_sign_commit(struct strbuf *buf, const char *keyid)
+{
+	struct strbuf sig = STRBUF_INIT;
+	int inspos, copypos;
+
+	/* find the end of the header */
+	inspos = strstr(buf->buf, "\n\n") - buf->buf + 1;
+
+	if (!keyid || !*keyid)
+		keyid = get_signing_key();
+	if (sign_buffer(buf, &sig, keyid)) {
+		strbuf_release(&sig);
+		return -1;
+	}
+
+	for (copypos = 0; sig.buf[copypos]; ) {
+		const char *bol = sig.buf + copypos;
+		const char *eol = strchrnul(bol, '\n');
+		int len = (eol - bol) + !!*eol;
+
+		if (!copypos) {
+			strbuf_insert(buf, inspos, gpg_sig_header, gpg_sig_header_len);
+			inspos += gpg_sig_header_len;
+		}
+		strbuf_insert(buf, inspos++, " ", 1);
+		strbuf_insert(buf, inspos, bol, len);
+		inspos += len;
+		copypos += len;
+	}
+	strbuf_release(&sig);
+	return 0;
+}
+
 static void handle_signed_tag(struct commit *parent, struct commit_extra_header ***tail)
 {
 	struct merge_remote_desc *desc;
@@ -975,13 +1012,14 @@
 
 int commit_tree(const char *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
-		const char *author)
+		const char *author, const char *sign_commit)
 {
 	struct commit_extra_header *extra = NULL, **tail = &extra;
 	int result;
 
 	append_merge_tag_headers(parents, &tail);
-	result = commit_tree_extended(msg, tree, parents, ret, author, extra);
+	result = commit_tree_extended(msg, tree, parents, ret,
+				      author, sign_commit, extra);
 	free_commit_extra_headers(extra);
 	return result;
 }
@@ -993,7 +1031,8 @@
 
 int commit_tree_extended(const char *msg, unsigned char *tree,
 			 struct commit_list *parents, unsigned char *ret,
-			 const char *author, struct commit_extra_header *extra)
+			 const char *author, const char *sign_commit,
+			 struct commit_extra_header *extra)
 {
 	int result;
 	int encoding_is_utf8;
@@ -1043,6 +1082,9 @@
 	if (encoding_is_utf8 && !is_utf8(buffer.buf))
 		fprintf(stderr, commit_utf8_warn);
 
+	if (sign_commit && do_sign_commit(&buffer, sign_commit))
+		return -1;
+
 	result = write_sha1_file(buffer.buf, buffer.len, commit_type, ret);
 	strbuf_release(&buffer);
 	return result;
diff --git a/commit.h b/commit.h
index 3745f12..d2c3e65 100644
--- a/commit.h
+++ b/commit.h
@@ -193,11 +193,11 @@
 
 extern int commit_tree(const char *msg, unsigned char *tree,
 		       struct commit_list *parents, unsigned char *ret,
-		       const char *author);
+		       const char *author, const char *sign_commit);
 
 extern int commit_tree_extended(const char *msg, unsigned char *tree,
 				struct commit_list *parents, unsigned char *ret,
-				const char *author,
+				const char *author, const char *sign_commit,
 				struct commit_extra_header *);
 
 extern struct commit_extra_header *read_commit_extra_headers(struct commit *);
diff --git a/notes-cache.c b/notes-cache.c
index 4c8984e..c36a960 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -56,7 +56,7 @@
 
 	if (write_notes_tree(&c->tree, tree_sha1))
 		return -1;
-	if (commit_tree(c->validity, tree_sha1, NULL, commit_sha1, NULL) < 0)
+	if (commit_tree(c->validity, tree_sha1, NULL, commit_sha1, NULL, NULL) < 0)
 		return -1;
 	if (update_ref("update notes cache", c->tree.ref, commit_sha1, NULL,
 		       0, QUIET_ON_ERR) < 0)
diff --git a/notes-merge.c b/notes-merge.c
index e9e4199..61cf18e 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -546,7 +546,7 @@
 		/* else: t->ref points to nothing, assume root/orphan commit */
 	}
 
-	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL))
+	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL, NULL))
 		die("Failed to commit notes tree to database");
 }