Brazil has the longest coastline in South America with more than 7491 km of hydrologically and topologically complex continental margin. Despite its extensive coast, the sea anemone fauna of the country is sparsely known with only 54 species recorded. Difficulties of accessing Brazil's highly diversified coast and the historical small number of dredging expeditions and sea anemone taxonomists have contributed to the limited knowledge of its sea anemone diversity particularly in deep waters. Most species recorded from Brazil correspond to large, conspicuous intertidal species and only five species have been recorded from depths greater than 200 m: two small burrowing edwardsiids and three hormathiids from the southern coast of Brazil. Here, we provide complete descriptions, images of external and internal anatomy, microanatomy, cnidae, and geographic distribution for 10 species, six off the coast of Brazil and four from the southern portion of the Mid-Atlantic Ridge, one of which is also present in the Southern Ocean. Four of the six species found in Brazil represent new records of geographically widespread species for the country and the remaining two endemic species have their geographic and bathymetric ranges significantly extended northward. As a result, the number of species known for the Brazilian coast is raised to 63 species, 14 of which are known from the deep sea. We also recorded four deep-sea species in the south Mid-Atlantic Ridge (SMAR), including Actinernus mercedae, sp. nov., and two new records for the South Atlantic. We found that the deep-sea fauna of Brazil shares two deep-sea species with Argentina and two with the North Atlantic. Thus, La Plata River acts as a filter for shallow water species between Brazil and Argentina but is less effective for deep-sea anemones. The diversity of sea anemones recorded for SMAR shows similarities to the southwestern Atlantic and Southern Ocean. Finally, our finding of representatives of Bolocera in the SMAR and Actinoscyphia off the Brazilian coast suggests that they are bipolar genera with tropical emergence.