Radiative cooling (RC) sees great developments in recent years due to its unique feature of sending waste heat to the cold universe without any additional energy consumption, which is extensively proved in many application scenarios, including its integration into solar installations. The comprehensive solar photovoltaic/thermal (PV/T) technology is becoming popular due to its multi-function and high overall efficiency. The integration of RC into a PV/T collector can further contribute to such merits by adding a night sky cooling function, so a PV/T-RC collector can produce electricity and heat during the daytime and provide cooling energy during the nighttime. Without any structural modification, a flat-plate PV/T collector with a typical glass cover is confirmed to be able to realize a good radiative cooling in the present study. A mathematic model for the nighttime performance evaluation of a typical PV/T module was developed to characterize the nocturnal cooling capacity of the module. Results suggest that the absorber plate can be cooled to nearly 9.5 °C below the ambient air over a consecutive five hours nighttime period. Further parametric studies were carried out to investigate the effect of some key structural and environmental parameters on the radiative cooling performance of the PV/T module. Under some favorable radiative cooling conditions, the absorber plate can realize a stagnation temperature of nearly 11 °C lower than the ambient temperature and reach a maximum cooling power of over 50 W/m2.