Calochortus monanthus is an extinct North American species of flowering plant in the lily family known by the common names single-flowered mariposa lily and Shasta River mariposa lily. It was endemic to northern California. It is presumed extinct, having been collected and documented once over a century ago and never found again. The single known specimen was collected by botanist Edward Lee Greene from a meadow on the banks of the Shasta River, near Yreka in Siskiyou County, California, in June of 1876. Calochortus monanthus had an unbranching stem and an inflorescence of a single erect, bell-shaped flower on a long peduncle. The flower had three sepals about 4 centimeters long and three toothed petals each between 4 and 5 centimeters. The petals were pinkish with a dark red spot at each base.