Washoe County School District Police Chief Mike Mieras tells KOLO-TV the incident happened Monday morning at a bus stop at the Park Vista apartment complex.
Officials at Marvin Moss Elementary School confiscated the knife from the boy's backpack and summoned the child's parent.
No injuries were reported. But parent Joyce Gonzalez tells KOLO the boy threatened her 7-year-old son's life.
A letter to parents informed them of the incident, but little information was released about the child or the possible consequences he might face.
Children under age eight can't face criminal prosecution in Nevada.