diff --git a/usr.bin/diff/diff.1 b/usr.bin/diff/diff.1
index 47d55a4ddfc5..eb6b16f2e63d 100644
--- a/usr.bin/diff/diff.1
+++ b/usr.bin/diff/diff.1
@@ -30,7 +30,7 @@
 .\"     @(#)diff.1	8.1 (Berkeley) 6/30/93
 .\" $FreeBSD$
 .\"
-.Dd February 12, 2020
+.Dd February 13, 2020
 .Dt DIFF 1
 .Os
 .Sh NAME
@@ -60,7 +60,7 @@
 .Op Fl -starting-file
 .Op Fl -speed-large-files
 .Op Fl -strip-trailing-cr
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Op Fl -unified
 .Op Fl I Ar pattern | Fl -ignore-matching-lines Ar pattern
@@ -88,7 +88,7 @@
 .Op Fl -speed-large-files
 .Op Fl -starting-file
 .Op Fl -strip-trailing-cr
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Fl C Ar number | -context Ar number
 .Ar file1 file2
@@ -113,7 +113,7 @@
 .Op Fl -speed-large-files
 .Op Fl -starting-file
 .Op Fl -strip-trailing-cr
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Fl D Ar string | Fl -ifdef Ar string
 .Ar file1 file2
@@ -139,7 +139,7 @@
 .Op Fl -speed-large-files
 .Op Fl -starting-file
 .Op Fl -strip-trailing-cr
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Fl U Ar number | Fl -unified Ar number
 .Ar file1 file2
@@ -170,7 +170,7 @@
 .Op Fl -show-c-function
 .Op Fl -speed-large-files
 .Op Fl -strip-trailing-cr
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Op Fl -unidirectional-new-file
 .Op Fl -unified
@@ -192,7 +192,7 @@
 .Op Fl -no-ignore-file-name-case
 .Op Fl -strip-trailing-cr
 .Op Fl -suppress-common-lines
-.Op Fl -tabsize
+.Op Fl -tabsize Ar number
 .Op Fl -text
 .Op Fl -width
 .Fl y | Fl -side-by-side
diff --git a/usr.bin/diff/diff.c b/usr.bin/diff/diff.c
index e2e65e520cb5..a4e86b7855a8 100644
--- a/usr.bin/diff/diff.c
+++ b/usr.bin/diff/diff.c
@@ -93,7 +93,7 @@ static struct option longopts[] = {
 	{ "no-ignore-file-name-case",	no_argument,		NULL,	OPT_NO_IGN_FN_CASE },
 	{ "normal",			no_argument,		NULL,	OPT_NORMAL },
 	{ "strip-trailing-cr",		no_argument,		NULL,	OPT_STRIPCR },
-	{ "tabsize",			optional_argument,	NULL,	OPT_TSIZE },
+	{ "tabsize",			required_argument,	NULL,	OPT_TSIZE },
 	{ "changed-group-format",	required_argument,	NULL,	OPT_CHANGED_GROUP_FORMAT},
 	{ "suppress-common-lines",	no_argument,		NULL,	OPT_SUPPRESS_COMMON },
 	{ NULL,				0,			0,	'\0'}
diff --git a/usr.bin/diff/tests/diff_test.sh b/usr.bin/diff/tests/diff_test.sh
index ba38befeccb8..b605c4733f09 100755
--- a/usr.bin/diff/tests/diff_test.sh
+++ b/usr.bin/diff/tests/diff_test.sh
@@ -10,6 +10,7 @@ atf_test_case side_by_side
 atf_test_case brief_format
 atf_test_case b230049
 atf_test_case Bflag
+atf_test_case tabsize
 
 simple_body()
 {
@@ -164,6 +165,16 @@ Bflag_body()
 	atf_check -s exit:1 -o file:"$(atf_get_srcdir)/Bflag_F.out" diff -B E F
 }
 
+tabsize_body()
+{
+	printf "\tA\n" > A
+	printf "\tB\n" > B
+
+	atf_check -s exit:1 \
+	    -o inline:"1c1\n<  A\n---\n>  B\n" \
+	    diff -t --tabsize 1 A B
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case simple
@@ -176,4 +187,5 @@ atf_init_test_cases()
 	atf_add_test_case brief_format
 	atf_add_test_case b230049
 	atf_add_test_case Bflag
+	atf_add_test_case tabsize
 }