This disclosure pertains to a post driver apparatus of simplified and compact construction. The apparatus can be easily attached to a vehicle equipped with a universal quick attach hitch and having auxiliary hydraulics, for example a farm or construction vehicle. The apparatus includes a hydraulic motor that is driven by the auxiliary hydraulics of the vehicle to which the apparatus is attached. The hydraulic motor rotates a drive wheel of the apparatus which is operatively connected with a weight piston of the apparatus. Rotation of the drive wheel lifts and then drops the weight piston on each rotation of the drive wheel. With the apparatus positioned on the top end of a post, each time the weight piston is dropped it impacts the top of the post and drives the post downwardly. The apparatus is equipped with a safety feature that prevents rotation of the drive wheel from operatively raising the weight piston until the apparatus is positioned on top of a post and exerts a force on the top end of the post.