A maser source in which the silicon monoxide (SiO) molecule is excited to maser action. SiO has many maser lines, which occur in groups at frequencies near 43, 86, 129 GHz, and so on. SiO masers are commonly found in circumstellar envelopes, close to the surface of the red giant where mass-loss originates. SiO masers are occasionally found in star-forming regions.