An adapter assembly for securing a luminaire to a niche formed in a wall of a pool or spa. The assembly comprises a mounting bracket having a rear portion shaped and sized to fit within the niche and a front portion comprising a peripheral flange sized to engage the wall surrounding the niche. The mounting bracket enables the luminaire to be secured thereto and be visible from a front side thereof. The assembly includes at least one clamping device attached to the rear portion of the mounting bracket and configured to secure the mounting bracket to the niche. In a preferred embodiment, the clamping device comprises a clamping arm and drive member. The clamping arm is pivotally connected to the mounting bracket such that the mounting bracket is drawn into the niche and the peripheral flange of the mounting bracket is drawn towards the wall surrounding the niche.