Calochortus tiburonensis (also known as the Tiburon Mariposa Lily) is a rare member of the genus Calochortus in the family Liliaceae. It was first brought to the attention of science in 1971, and written up in the botanical literature in 1973. Calochortus tiburonensis is endemic to a single serpentine outcrop in California's Marin County. It grows on serpentine-derived soil atop this outcrop in grasslands from 0-200 meter. on Ring Mountain. It is considered threatened due to loss of surrounding habitat to recreational activities, to competition from invasive species, to its proximity to a dense population center, and to its confinement to a single population of plants. The population grows on land owned by the Marin County Open Space District. Calochortus tiburonensis is a perennial growing from a bulb to 10-60 centimeter in height with a single leaf. It produces anywhere from 2 to 7 erect flowers annually on a branching stem. The ciliate, light yellow-green petals are streaked with purplish-brown. When mature, ovaries form capsules full of small, dark brown seeds.