Amaranthus greggii, also known as Gregg's amaranth or Josiah amaranth, is a glabrous annual flowering plant native to Texas, Louisiana, and Mexico. The plant can grow up to 1 m (3 ft) in height. It is found in sand dunes and near sea beaches. The species name greggii honors Josiah Gregg (1806–1850), a merchant, explorer, naturalist, and author of the American Southwest and Northern Mexico.

According to J. D. Sauer (1972b), Amaranthus myrianthus Standley most probably is a hybrid between A. arenicola and A. greggii. The seeds of A.greggii are dark brown and shiny, with 1.2-1.7 mm in diameter.

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