Calochortus monanthus is a presumed 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 1876.

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