Clarkia williamsonii is a species of flowering plant in the evening primrose family known by the common name Fort Miller clarkia. It is endemic to California, where it is known from the forests and woodlands of the northern and central Sierra Nevada foothills. This is an erect annual herb with linear to lance-shaped leaves each a few centimeters long. The flower cluster produces opening flowers and buds which are closed except for the tips, where the sepals do not fuse. The sepals all separate or remain fused in pairs as the flower blooms. Each fan-shaped petal is up to 3 centimeters long and is usually lavender with a white area and a purple spot in the middle. The petals are occasionally solid deep red.