Doppler observations from Keck Observatory reveal the presence of a planet with M sin i of 2.1 MJup orbiting the M3V star HIP 79431. This is the sixth giant planet to be detected in Doppler surveys of M dwarfs and it is one of the most massive planets discovered around an M dwarf star. The planet has an orbital period of 111.7 days and an orbital eccentricity of 0.29. The host star is metal rich, with an estimated [Fe/H] = +0.4. This is the first planet to emerge from our new survey of 1600 M-to-K dwarf stars.Based on observations obtained at the W. M. Keck Observatory, which is operated by the University of California.