diff --git a/lib/libc/sys/kqueue.2 b/lib/libc/sys/kqueue.2
--- a/lib/libc/sys/kqueue.2
+++ b/lib/libc/sys/kqueue.2
@@ -250,6 +250,21 @@
 See
 .Sx RETURN VALUES
 below.
+.It Dv EV_KEEPUDATA
+Causes
+.Fn kevent
+to leave unchanged any
+.Fa udata
+associated with an existing event.  This allows other aspects of the
+event to be modified without requiring the caller to know the
+.Fa udata
+value presently associated.
+This is especially useful with
+.Dv NOTE_TRIGGER
+or flags like
+.Dv EV_ENABLE.
+This flag may not be used with
+.Dv EV_ADD.
 .El
 .Pp
 The predefined system filters are listed below.
diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1441,6 +1441,13 @@
 		return EINVAL;
 
 	if (kev->flags & EV_ADD) {
+		/* Reject an invalid flag pair early */
+		if (kev->flags & EV_KEEPUDATA) {
+			tkn = NULL;
+			error = EINVAL;
+			goto done;
+		}
+
 		/*
 		 * Prevent waiting with locks.  Non-sleepable
 		 * allocation failures are handled in the loop, only
@@ -1629,7 +1636,8 @@
 	kn_enter_flux(kn);
 	KQ_UNLOCK(kq);
 	knl = kn_list_lock(kn);
-	kn->kn_kevent.udata = kev->udata;
+	if ((kev->flags & EV_KEEPUDATA) == 0)
+		kn->kn_kevent.udata = kev->udata;
 	if (!fops->f_isfd && fops->f_touch != NULL) {
 		fops->f_touch(kn, kev, EVENT_REGISTER);
 	} else {
diff --git a/sys/sys/event.h b/sys/sys/event.h
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -163,6 +163,7 @@
 #define EV_ENABLE	0x0004		/* enable event */
 #define EV_DISABLE	0x0008		/* disable event (not reported) */
 #define EV_FORCEONESHOT	0x0100		/* enable _ONESHOT and force trigger */
+#define EV_KEEPUDATA	0x0200		/* do not update the udata field */
 
 /* flags */
 #define EV_ONESHOT	0x0010		/* only report one occurrence */