diff --git a/share/man/man9/KASSERT.9 b/share/man/man9/KASSERT.9
--- a/share/man/man9/KASSERT.9
+++ b/share/man/man9/KASSERT.9
@@ -40,7 +40,15 @@
 .Pp
 .In sys/param.h
 .In sys/systm.h
-.Fn KASSERT expression msg
+.Fn KASSERT expression
+.Fn KASSERT expression fmt ...
+.Fo KASSERT
+.Fa expression
+.Ns , (
+.Fa fmt
+.Fa ...
+.Ns )
+.Ns Fc
 .Fn MPASS expression
 .Sh DESCRIPTION
 Assertions are widely used within the
@@ -69,12 +77,23 @@
 This terminates the running system at the point of the error, possibly dropping
 into the kernel debugger or initiating a kernel core dump.
 The second argument,
-.Fa msg ,
+.Fa fmt ,
 is a
 .Xr printf 9
-format string and its arguments,
-enclosed in parentheses.
+format string followed by its arguments.
 The formatted string will become the panic string.
+If no
+.Fa fmt
+argument is provided, a default string including the current file and line is used.
+.Pp
+The third form of
+.Fn KASSERT
+which encloses
+.Fa fmt
+and its arguments in a nested pair of parentheses is deprecated but must currently
+been used when
+.Fa fmt
+does not accept additional arguments.
 .Pp
 In a kernel that is built without
 .Cd "options INVARIANTS" ,
@@ -135,8 +154,8 @@
 {
 
 	KASSERT((fp->foo_flags & FOO_ACTIVE) == 0,
-	    ("%s: fp %p is still active, flags=%x", __func__, fp,
-	    fp->foo_flags));
+	    "%s: fp %p is still active, flags=%x", __func__, fp,
+	    fp->foo_flags);
 	...
 }
 .Ed
@@ -150,7 +169,7 @@
 .Pp
 The assertion
 .Bd -literal -offset indent
-MPASS(td == curthread);
+KASSERT(td == curthread);
 .Ed
 .Pp
 located on line 87 of a file named foo.c would generate the following panic
@@ -179,11 +198,20 @@
 .Pp
 According to the guidelines above, this would be correctly expressed as:
 .Bd -literal -offset indent
-MPASS(td == curthread);
+KASSERT(td == curthread);
 KASSERT(sz >= SIZE_MIN && sz <= SIZE_MAX,
     ("invalid size argument: %u", sz));
 .Ed
 .Sh HISTORY
+The third form of the
+.Nm KASSERT
+macro first appeared in
+.Fx 3.0 .
+The first and second forms of
+.Nm KASSERT
+macro first appeared in
+.Fx 15.0 .
+.Pp
 The
 .Nm MPASS
 macro first appeared in
diff --git a/sys/sys/kassert.h b/sys/sys/kassert.h
--- a/sys/sys/kassert.h
+++ b/sys/sys/kassert.h
@@ -146,12 +146,30 @@
  * Kernel assertion; see KASSERT(9) for details.
  */
 #if (defined(_KERNEL) && defined(INVARIANTS)) || defined(_STANDALONE)
-#define	KASSERT(exp,msg) do {						\
+#define	KASSERTN(exp, ...) do {						\
+	if (__predict_false(!(exp)))					\
+		kassert_panic(__VA_ARGS__);				\
+} while (0)
+#define	KASSERT2(exp, msg) do {						\
 	if (__predict_false(!(exp)))					\
 		kassert_panic msg;					\
 } while (0)
+#define	KASSERT1(exp) do {						\
+	if (__predict_false(!(exp)))					\
+		kassert_panic("Assertion %s failed at %s:%d", #exp,	\
+		    __FILE__, __LINE__);				\
+} while (0)
+
+#define _KASSERT_MACRO(exp, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10,	\
+    NAME, ...)								\
+	NAME
+
+#define KASSERT(...)							\
+	_KASSERT_MACRO(__VA_ARGS__, KASSERTN, KASSERTN, KASSERTN,	\
+	    KASSERTN, KASSERTN, KASSERTN, KASSERTN, KASSERTN, KASSERTN, \
+	    KASSERT2, KASSERT1)(__VA_ARGS__)
 #else /* !(KERNEL && INVARIANTS) && !_STANDALONE */
-#define	KASSERT(exp,msg) do { \
+#define	KASSERT(...) do { \
 } while (0)
 #endif /* (_KERNEL && INVARIANTS) || _STANDALONE */
 
@@ -167,7 +185,7 @@
 #define MPASS2(ex, what)	MPASS4(ex, what, __FILE__, __LINE__)
 #define MPASS3(ex, file, line)	MPASS4(ex, #ex, file, line)
 #define MPASS4(ex, what, file, line)					\
-	KASSERT((ex), ("Assertion %s failed at %s:%d", what, file, line))
+	KASSERT((ex), "Assertion %s failed at %s:%d", what, file, line)
 
 /*
  * Assert that a pointer can be loaded from memory atomically.