The Sm2SiO5 ceramics were synthesized and their microwave dielectric properties were investigated. The pure monoclinic Sm2SiO5 phase could be obtained when the molar ratio of Sm2O3/SiO2 was 1:1.05 at 1350°C. A small amount of hexagonal Sm4(SiO4)3 second phase occurred as temperature increased. The relative density of Sm2SiO5 ceramics increased with increasing temperature. The Sm2SiO5 ceramics sintered at 1500°C exhibited microwave dielectric properties: a dielectric constant (εr) of 8.44, a quality factor (Q × f) of 64 000 GHz, and a temperature coefficient of resonant frequency (τf) of −37 ppm/°C. The Sm2SiO5 ceramics have a wide sintering temperature region and small negative τf values. They are promising candidate materials for millimeter-wave devices.