Lacydonia Marion & Bobretzky in Marion, 1874 is the only known genus in Lacydoniidae Bergstrom, 1914, which is composed of small polychaetes scarcely sampled. Studies on the biology and ecology of this group are rare and most species descriptions are based on one or a few specimens. There are 13 nominal species from different parts of the world. We revise herein the morphological characters used for species description on the basis of a lacydoniid assemblage collected in Campos Basin, Southern Brazil. Material was obtained in two sampling campaigns during the Project Habitats/Petrobras - Heterogeneidade Ambiental da Bacia de Campos, coordinated by CENPES/Petrobras. Lacydoniids were collected from the continental shelf, canyons and slope, at depths from 25 to 2500 m. This polychaete family is recorded for the first time in Brazilian waters, three new species are described and all others are new records to the South Atlantic. The new species are Lacydonia anapaulae sp. nov., L. brasiliensis sp. nov. and L. jacki sp. nov. Newly recorded species are L. cirrata (Hartman & Fauchald, 1971), L. laureci Laubier, 1975, L. miranda Marion & Bobretzky in Marion, 1874, L. oculata (Hartman, 1967) and L. cf. papillata Uschakov, 1958. An updated key to all described species of Lacydonia is included.
