We have investigated structural, optical and magnetic properties of Sm-doped GaN thin films grown by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction and X-ray diffraction reveal that Ga1− x Sm x N films with a SmN mole fraction of ∼8% or below are grown on GaN templates without segregation of any secondary phases. With increasing SmN mole fraction, the c-axis lattice parameter of the GaSmN films linearly increases. GaSmN films with low Sm concentrations exhibit inner-4f transitions of Sm3+ in photoluminescence spectra. The present findings show that Sm atoms are substituted for some Ga atoms as trivalent ions (Sm3+). The Ga1− x Sm x N films display hysteresis loops in magnetization versus external magnetic field (M–H) curves even at 300 K. We will discuss the origin of these features together with the corresponding temperature dependences of magnetization.