A post driver for driving hollow posts, especially plastic posts, has an elongated central guide shaft and a hollow outer sleeve. The guide shaft has a length and diameter sized to pass fairly freely through but generally fill the post's hollow core to protect against the post buckling during pounding strokes. The outer sleeve annularly surrounds a substantial upper portion of the post for similar reasons. The outer sleeve transmits pounding strikes to a stop formation that rests stationary on the post's top end. That way, a user manually applying pounding strokes to the stop formation's upper surface transmits force to not only the post's top end but also the guide shaft's bottom end. Optionally, the guide shaft's bottom end is tipped and sticks out a little in order to pierce a pilot hole in the ground, and clear a path, for the post to follow.