Stable radio-photoluminescence (RPL) is observed in NaMgF3(Sm) after X-ray irradiation due to valence conversion of Sm3+ to Sm2+. This valence conversion can be reversed by deep UV (254 nm) illumination. It is likely that this reversible process occurs via defects near some of the Sm3+ sites that act as hole traps. The ratio of the Sm2+/Sm3+ RPL emissions follows an exponential-type curve dependence on X-ray dose where the ratio saturates above similar to 500 Gy. Optically stimulated luminescence (OSL) is also observed but only from Sm3+ and primarily from electrons trapped at F-centers. The OSL response is demonstrated to be linear up to 100 Gy. Thus, NaMgF3(Sm) has potential as a dosimeter where individual irradiations can be monitored via OSL and the total cumulative dose via non-destructive RPL readout. The RPL can be optically reset when required.