diff --git a/builtin/commit.c b/builtin/commit.c
index 2032635e6e..eba1377eb3 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1495,7 +1495,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	if (amend) {
-		extra = read_commit_extra_headers(current_head);
+		const char *exclude_gpgsig[2] = { "gpgsig", NULL };
+		extra = read_commit_extra_headers(current_head, exclude_gpgsig);
 	} else {
 		struct commit_extra_header **tail = &extra;
 		append_merge_tag_headers(parents, &tail);
diff --git a/commit.c b/commit.c
index 3d3ccaea06..35af4988f0 100644
--- a/commit.c
+++ b/commit.c
@@ -981,14 +981,15 @@ static void add_extra_header(struct strbuf *buffer,
 		strbuf_addch(buffer, '\n');
 }
 
-struct commit_extra_header *read_commit_extra_headers(struct commit *commit)
+struct commit_extra_header *read_commit_extra_headers(struct commit *commit,
+						      const char **exclude)
 {
 	struct commit_extra_header *extra = NULL;
 	unsigned long size;
 	enum object_type type;
 	char *buffer = read_sha1_file(commit->object.sha1, &type, &size);
 	if (buffer && type == OBJ_COMMIT)
-		extra = read_commit_extra_header_lines(buffer, size);
+		extra = read_commit_extra_header_lines(buffer, size, exclude);
 	free(buffer);
 	return extra;
 }
@@ -1002,7 +1003,23 @@ static inline int standard_header_field(const char *field, size_t len)
 		(len == 8 && !memcmp(field, "encoding ", 9)));
 }
 
-struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, size_t size)
+static int excluded_header_field(const char *field, size_t len, const char **exclude)
+{
+	if (!exclude)
+		return 0;
+
+	while (*exclude) {
+		size_t xlen = strlen(*exclude);
+		if (len == xlen &&
+		    !memcmp(field, *exclude, xlen) && field[xlen] == ' ')
+			return 1;
+		exclude++;
+	}
+	return 0;
+}
+
+struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, size_t size,
+							   const char **exclude)
 {
 	struct commit_extra_header *extra = NULL, **tail = &extra, *it = NULL;
 	const char *line, *next, *eof, *eob;
@@ -1028,7 +1045,8 @@ struct commit_extra_header *read_commit_extra_header_lines(const char *buffer, s
 		if (next <= eof)
 			eof = next;
 
-		if (standard_header_field(line, eof - line))
+		if (standard_header_field(line, eof - line) ||
+		    excluded_header_field(line, eof - line, exclude))
 			continue;
 
 		it = xcalloc(1, sizeof(*it));
diff --git a/commit.h b/commit.h
index 9b56ae6814..154c0e34ff 100644
--- a/commit.h
+++ b/commit.h
@@ -200,8 +200,8 @@ extern int commit_tree_extended(const struct strbuf *msg, unsigned char *tree,
 				const char *author, const char *sign_commit,
 				struct commit_extra_header *);
 
-extern struct commit_extra_header *read_commit_extra_headers(struct commit *);
-extern struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len);
+extern struct commit_extra_header *read_commit_extra_headers(struct commit *, const char **);
+extern struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 
 extern void free_commit_extra_headers(struct commit_extra_header *extra);
 
diff --git a/gpg-interface.c b/gpg-interface.c
index 18630ff8da..09ab64aa24 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -95,10 +95,7 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 
 /*
  * Run "gpg" to see if the payload matches the detached signature.
- * gpg_output_to tells where the output from "gpg" should go:
- *   < 0: /dev/null
- *   = 0: standard error of the calling process
- *   > 0: the specified file descriptor
+ * gpg_output, when set, receives the diagnostic output from GPG.
  */
 int verify_signed_buffer(const char *payload, size_t payload_size,
 			 const char *signature, size_t signature_size,
diff --git a/log-tree.c b/log-tree.c
index 5cac6bfcd4..c719a6e385 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -404,13 +404,27 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 	*extra_headers_p = extra_headers;
 }
 
+static void show_sig_lines(struct rev_info *opt, int status, const char *bol)
+{
+	const char *color, *reset, *eol;
+
+	color = diff_get_color_opt(&opt->diffopt,
+				   status ? DIFF_WHITESPACE : DIFF_FRAGINFO);
+	reset = diff_get_color_opt(&opt->diffopt, DIFF_RESET);
+	while (*bol) {
+		eol = strchrnul(bol, '\n');
+		printf("%s%.*s%s%s", color, (int)(eol - bol), bol, reset,
+		       *eol ? "\n" : "");
+		bol = (*eol) ? (eol + 1) : eol;
+	}
+}
+
 static void show_signature(struct rev_info *opt, struct commit *commit)
 {
 	struct strbuf payload = STRBUF_INIT;
 	struct strbuf signature = STRBUF_INIT;
 	struct strbuf gpg_output = STRBUF_INIT;
 	int status;
-	const char *color, *reset, *bol, *eol;
 
 	if (parse_signed_commit(commit->object.sha1, &payload, &signature) <= 0)
 		goto out;
@@ -421,17 +435,7 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
 	if (status && !gpg_output.len)
 		strbuf_addstr(&gpg_output, "No signature\n");
 
-	color = diff_get_color_opt(&opt->diffopt,
-				   status ? DIFF_WHITESPACE : DIFF_FRAGINFO);
-	reset = diff_get_color_opt(&opt->diffopt, DIFF_RESET);
-
-	bol = gpg_output.buf;
-	while (*bol) {
-		eol = strchrnul(bol, '\n');
-		printf("%s%.*s%s%s", color, (int)(eol - bol), bol, reset,
-		       *eol ? "\n" : "");
-		bol = (*eol) ? (eol + 1) : eol;
-	}
+	show_sig_lines(opt, status, gpg_output.buf);
 
  out:
 	strbuf_release(&gpg_output);
@@ -439,6 +443,90 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
 	strbuf_release(&signature);
 }
 
+static int which_parent(const unsigned char *sha1, const struct commit *commit)
+{
+	int nth;
+	const struct commit_list *parent;
+
+	for (nth = 0, parent = commit->parents; parent; parent = parent->next) {
+		if (!hashcmp(parent->item->object.sha1, sha1))
+			return nth;
+		nth++;
+	}
+	return -1;
+}
+
+static int is_common_merge(const struct commit *commit)
+{
+	return (commit->parents
+		&& commit->parents->next
+		&& !commit->parents->next->next);
+}
+
+static void show_one_mergetag(struct rev_info *opt,
+			      struct commit_extra_header *extra,
+			      struct commit *commit)
+{
+	unsigned char sha1[20];
+	struct tag *tag;
+	struct strbuf verify_message;
+	int status, nth;
+	size_t payload_size, gpg_message_offset;
+
+	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), sha1);
+	tag = lookup_tag(sha1);
+	if (!tag)
+		return; /* error message already given */
+
+	strbuf_init(&verify_message, 256);
+	if (parse_tag_buffer(tag, extra->value, extra->len))
+		strbuf_addstr(&verify_message, "malformed mergetag\n");
+	else if (is_common_merge(commit) &&
+		 !hashcmp(tag->tagged->sha1,
+			  commit->parents->next->item->object.sha1))
+		strbuf_addf(&verify_message,
+			    "merged tag '%s'\n", tag->tag);
+	else if ((nth = which_parent(tag->tagged->sha1, commit)) < 0)
+		strbuf_addf(&verify_message, "tag %s names a non-parent %s\n",
+				    tag->tag, tag->tagged->sha1);
+	else
+		strbuf_addf(&verify_message,
+			    "parent #%d, tagged '%s'\n", nth + 1, tag->tag);
+	gpg_message_offset = verify_message.len;
+
+	payload_size = parse_signature(extra->value, extra->len);
+	if ((extra->len <= payload_size) ||
+	    (verify_signed_buffer(extra->value, payload_size,
+				  extra->value + payload_size,
+				  extra->len - payload_size,
+				  &verify_message) &&
+	     verify_message.len <= gpg_message_offset)) {
+		strbuf_addstr(&verify_message, "No signature\n");
+		status = -1;
+	}
+	else if (strstr(verify_message.buf + gpg_message_offset,
+			": Good signature from "))
+		status = 0;
+	else
+		status = -1;
+
+	show_sig_lines(opt, status, verify_message.buf);
+	strbuf_release(&verify_message);
+}
+
+static void show_mergetag(struct rev_info *opt, struct commit *commit)
+{
+	struct commit_extra_header *extra, *to_free;
+
+	to_free = read_commit_extra_headers(commit, NULL);
+	for (extra = to_free; extra; extra = extra->next) {
+		if (strcmp(extra->key, "mergetag"))
+			continue; /* not a merge tag */
+		show_one_mergetag(opt, extra, commit);
+	}
+	free_commit_extra_headers(to_free);
+}
+
 void show_log(struct rev_info *opt)
 {
 	struct strbuf msgbuf = STRBUF_INIT;
@@ -550,8 +638,10 @@ void show_log(struct rev_info *opt)
 		}
 	}
 
-	if (opt->show_signature)
+	if (opt->show_signature) {
 		show_signature(opt, commit);
+		show_mergetag(opt, commit);
+	}
 
 	if (!commit->buffer)
 		return;
diff --git a/t/t7510-signed-commit.sh b/t/t7510-signed-commit.sh
index 30401ced07..1d3c56fe61 100755
--- a/t/t7510-signed-commit.sh
+++ b/t/t7510-signed-commit.sh
@@ -24,7 +24,8 @@ test_expect_success GPG 'create signed commits' '
 	echo 4 >file && test_tick && git commit -a -m "fourth unsigned" &&
 	git tag fourth-unsigned &&
 
-	test_tick && git commit --amend -S -m "fourth signed"
+	test_tick && git commit --amend -S -m "fourth signed" &&
+	git tag fourth-signed
 '
 
 test_expect_success GPG 'show signatures' '
@@ -68,4 +69,12 @@ test_expect_success GPG 'detect fudged signature with NUL' '
 	! grep "Good signature from" actual2
 '
 
+test_expect_success GPG 'amending already signed commit' '
+	git checkout fourth-signed^0 &&
+	git commit --amend -S --no-edit &&
+	git show -s --show-signature HEAD >actual &&
+	grep "Good signature from" actual &&
+	! grep "BAD signature from" actual
+'
+
 test_done