This paper reports on the first implementation of a single-photon avalanche diode (SPAD) in standard silicon on insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology. The SPAD is realized in a circular shape, and it is based on a P(+)/N-well junction along with a P-well guard-ring structure formed by lateral diffusion of two closely spaced N-well regions. The SPAD electric-field profile is analyzed by means of simulation to predict the breakdown voltage and the effectiveness of premature edge breakdown. Measurements confirm these predictions and also provide a complete characterization of the device, including current-voltage characteristics, dark count rate (DCR), photon detection probability (PDP), afterpulsing probability, and photon timing jitter. The SOI CMOS SPAD has a PDP above 25% at 490-nm wavelength and, thanks to built-in optical sensitivity enhancement mechanisms, it is as high as 7.7% at 850-nm wavelength. The DCR is 244 Hz/μm2, and the afterpulsing probability is less than 0.1% for a dead time longer than 200 ns. The SPAD exhibits a timing response without exponential tail and provides a remarkable timing jitter of 65 ps (FWHM). The new device is well suited to operate in backside illumination within complex three-dimensional (3D) integrated circuits, thus contributing to a great improvement of fill factor and jitter uniformity in large arrays.