The genus Lyonsia Turton, 1822 is represented in the southwestern Atlantic Ocean by three species: Lyonsia hyalina (Conrad, 1831), which is very common on the east coast of the U.S.A. and has a few isolated records from Brazil and Uruguay; Lyonsia malvinensis d'Orbigny, 1846, known only from the Falkland (Malvinas) Islands; and Lyonsia alvarezii d'Orbigny, 1846, restricted to southern Brazil, Uruguay and Argentina. Some authors consider L. alvarezii a synonym of L. hyalina. Study of material deposited in malacological collections (including types) allowed a redescription of L. alvarezii and its comparison with L. hyalina. Lyonsia alvarezii is characterized by markedly inequilateral valves, with the umbo well dislocated to the anterior region. It has considerable intraspecific variation, mainly in the ventral outline, from almost straight to sinoidal, and at the umbo, from pointed to slightly rounded. The arrangement of spinules on the outer surface of the shell is irregular, usually consisting of three to five radial rows between each pair of radial lines. Previous records of L. hyalina from southern Brazil and Uruguay were based on misidentifications of shells of L. alvarezii and of Lyonsia celeste n. sp. The latter is diagnosed by its elliptical shell, with the anterior, posterior and ventral outline rounded; its subequilateral valves; and the shell spinules restricted to the umbo region of the shell surface. Four species occur on the coast of Canada and the U.S.A: L. arenosa (Møller, 1842), L. granulifera Verrill and Bush, 1898, L. hyalina and L. floridana Conrad, 1849. Three are restricted to the southwestern Atlantic: Lyonsia alvarezii has a wide distribution, from shallow waters in northeastern (∼3°S) and southeastern Brazil (∼23°S) to Argentina (∼41°S); L. malvinensis is restricted to the Falkland Islands; and Lyonsia celeste n. sp. is known only from Uruguay and Argentina.