Hesperolinon congestum, or Marin Dwarf Flax, is an annual herb, which is known to occur only in San Mateo, San Francisco and Marin County, California, USA. This plant occurs chiefly on serpentine soils, especially in dry native bunch grasses, chaparral or other grasslands at elevations less than 200 meters. The flowers are congested at the tips of the dichotomously branching stems. H. congestum is in flower between April and July. The outlook for this plant depends on survival of only about twenty small colonies, most of which are not actively managed for protection, even though the species is federally and state-listed as threatened. This species is also less commonly known as Marin Western Flax.