Random telegraph signal (RTS) behavior is reported and characterized in the dark count rate of single-photon avalanche Diodes (SPADs). The RTS is observed in a SPAD fabricated in 0.8-mu m CMOS technology and in four proton-irradiated SPADs designed and fabricated in 0.35-mu m CMOS technology. The RTS characteristics are evaluated experimentally and verified theoretically with respect to bias and temperature.