Lassen clarkia is an attractive annual wildflower with simple to open-branched stems from 10-90 cm high. The herbage of stems and leaves varies from nearly glabrous to mostly covered with numerous, short, appressed hairs. The leaves are erect or ascending, linear in shape, and from 2-5 cm long and 1-5 mm wide. The base of the leaf is subsessile or short-petiolate.
The flowers are found at the top of the stem, arising from the leaf axils. The tip of the inflorescence and the unopened buds are recurved to the side or downwards, only becoming erect as the flowers open. The sepals are joined and turned to one side beneath the flower. They measure from 6-14 mm long. The hypanthium is 1-3 mm long. The 4 pink to lavender petals lack spots, are obovate in shape, and measure from 8-16 mm long. The floral tube is 2.5-5 mm long. The cream colored stigma lobes are 1-1.5 mm long while the style is shorter than the stamens. The anthers are all alike, ranging from 2-6 mm long, about equal to or longer than the filaments. At flowering, the ovary is longitudinally 8-grooved. The cylindrical capsules are straight, 2.5-4 cm long, and widest at the middle, tapering gradually to each end.
Lassen clarkia is similar to herald-of-summer, Clarkia amoena as well as slender godetia Clarkia gracilis.
Lassen clarkia is found in dry, open woods between the elevations of 1300-2000 meters.
Lassen clarkia is found from Deschutes County in central Oregon south to central California and east to the Steens Mt. in southeastern Oregon and Washoe and Storey Counties in Nevada.