Lilaeopsis masonii is a species of flowering plant in the carrot family known by the common names mudflat quillplant and Mason's lilaeopsis. It is endemic to California, where it is known only from the Sacramento-San Joaquin River Delta and nearby shores of San Francisco Bay. It is a plant of freshwater and brackish marshes and other estuary habitat. The plant is rare overall, limited in distribution to about 80 populations in a single network of water bodies, but it is locally abundant in some areas. It is a common bayside plant in Suisun Marsh.