Abstract: The present invention is a post support for use in stabilizing a post during installation. The invention features a support body housing a rack-and-pinion system that incorporates an extendable arm, so that the support may be mounted upon the post and extended to interface with the walls of the installation hole. I an alternate embodiment, the extensible arm and support body are in threaded engagement and the arm is simply screwed out. Dual armed embodiments are also possible. Another embodiment features the support and arm interfacing each other with inclined planes which force the arm downward and outward with the force of gravity. With all embodiments, a bearing plate is present for contacting the wall of the installation hole at the terminal end of the arm. The support is ideally made of rigid plastic or other suitable material, allowing for a low cost and eventual sacrifice of the support as it is entombed in foundational materials for long term post support.