This study provides a revision of the Magellanic and Antarctic species of Thyasira and Parathyasira. The status of all species previously reported from the area is revised and a new species described. Five species are recognized as valid: Thyasira falklandica (Smith, 1885); Thyasira scotiana, n. sp.; Thyasira debilis (Thiele, 1912), new combination; Parathyasira magellanica (Dall, 1901), new combination; and Parathyasira dearborni (Nicol, 1965), new combination. Cryptodon fuegiensis Dall, 1890, previously reported under the genus Thyasira, is reallocated into Conchocele Gabb, 1866. Loripes pertenuis Smith, 1881, and Thyasira bongraini Lamy, 1911, are considered nomen dubia.