The phorid fly genus Megaselia Rondani is a large, poorly-known taxon whose species are found worldwide. A new species of Megaselia Rondani, M.simunorum, is described from both urban and rural sites in southern California. With a large area of white colour on the posterior part of the abdominal dorsum, it closely resembles the much more common species M.sulphurizona, but M.simunorum has distinctly thicker ventral setae on the abdomen and a differently-shaped white spot.