We report on the world's first back-illuminated 3D-stacked single-photon avalanche diode (SPAD) in 45nm CMOS technology. This SPAD achieves a dark count rate of 55.4cps/μm 2 , a maximum photon detection probability of 31.8% at 600nm, over 5% in the 420-920nm wavelength range, and timing jitter of 107.7ps at 2.5V excess bias voltage and room temperature. To the best of our knowledge, these are the best results ever reported for any back-illuminated 3D-stacked SPAD technology.