Astragalus agnicidus is a rare species of milkvetch known by the common name Humboldt County milkvetch. It is endemic to northern California, where it is known only from two populations in Humboldt County and one in Mendocino County. The plant was undescribed until the 1950s. Until then it was known only from one 8-acre (32,000 m2) ranch in Humboldt County, where sheep ranchers blamed the unnamed species for the deaths of their animals, which may have eaten it. They eradicated the plant from their land, and by the time it was formally described and named in 1957 it was thought to be extinct. The species name given the plant, agnicidus, means "lamb-killer".