A clamp for securing a pole top luminaire to the top of a support post. A fitter clamp is disposed inside of the support post, while the housing of the luminaire includes a base portion which extends from the luminaire downward surrounding the outer circumference of the support post. Threaded fasteners extend through the housing base portion to engage the fitter clamp, causing the clamp to expand and compressively engage the support post between the fitter clamp and the luminaire housing base portion.