A flip-up aiming sight for a firearm includes a mounting base having a top platform, an illuminator embedded under the platform of the mounting base, and a flip up lens module which includes a lens frame pivotally coupled at a front side of the mounting base and a reflecting lens encirclingly mounted at the lens frame. The flip up lens module is arranged to pivotally fold between a folded position and an unfolded position. At the folded position, the lens frame is rearwardly and downwardly folded flat on the platform of the mounting base, and at the unfolded position, the lens frame is frontwardly and upwardly moved from the platform, such that the illuminator generates an aiming point on the reflecting lens.