Acmispon haydonii, synonyms Lotus haydonii and Syrmatium haydonii, is a species of legume native to California It is known by the common names pygmy lotus, rock bird's-foot trefoil and Haydon's lotus.

It is native to the dry mountain slopes and deserts of southern California, mainly the deserts of eastern San Diego County, where it grows in scrub and woodland habitat. It is a small bushy perennial herb spreading with mostly naked, slender stems. The sparse leaves are made up of usually three tiny leaflets and are deciduous. The inflorescence is generally made up of one or two yellow to reddish pealike flowers each under a centimeter long. The fruit is a curved legume pod also under a centimeter long.

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