This work presents a simulation and experimental study of the photon detection probability (PDP) enhancement in CMOS single-photon avalanche diodes (SPADs) using an anti-reflection coating (ARC) above the sensitive area. It is shown how the ARC layer can improve the PDP, not only by improving the optical transmission, but also by eliminating the penetration of the standing wave into a shallow region close to the silicon surface, where the multiplication region of the SPAD is formed. Furthermore, the appropriate ARC layer thickness corresponding to maximum PDP enhancement at different wavelengths over the visible spectrum is extracted to provide insight regarding the ARC selection if different ARC thicknesses are available within the CMOS process.