The luminescence spectra and decay times of CaO:Sn2+ are studied in the temperature range between 4.2 and 295 K. The emission spectra show one emission band with a maximum at 490 nm at 4.2 K. In the diffuse reflection spectra and the luminescence excitation spectra the A, B, and C bands are observed at 365, 324, and 294 nm, respectively. CaO:Sn2+ shows a very long after‐glow when excited at room temperature. The observed temperature dependence of the decay time may be fitted to a three level scheme leading to the parameter values τ0 = 350 μs, τ1 = 0.7 μs, and ΔE = 43 cm−1. A comparison is made with the luminescence of Sn2+ and Pb2+ in other calcium chalcogenides.