Chenopodium littoreum is described as new. It had been incorrectly cited in the past as C. carnosulum Moq. var. patagonicum (Phil.) Wahl, a variety of the South American C. carnosulum. However, C. littoreum differs from the C. carnosulum complex in having narrowly elliptic to lanceolate and mostly unlobed leaves, consistently five stamens per flower, and seeds that are invariably horizontal. Chenopodium littoreum is similar to another South American taxon, C. patagonicum Phil. ( = C. philippianum Aellen), but the latter differs in having basally lobed leaves, sepals fused above the middle, and generally one or two (rarely five) stamens. Chenopodium littoreum has a range currently known only from coastal dunes of San Luis Obispo Co. and Santa Barbara Co. of the Central Coast of California, plus a single historic collection from Los Angeles Co. of the South Coast of California.