Diadumenidae is a monogeneric family comprised of 10 valid species in the genus Diadumene. Although these species are distributed in all but the Southern Ocean, Diadumene lineata is a cosmopolitan species and one of the most widely distributed marine organisms. Recent phylogenetic analyses with multiple species have recovered Diadumenidae as a monophyletic group with high support. Although generic placement is straightforward, species-level identifications within Diadumene are difficult because species are defined by a mosaic of characters that vary in degree rather than kind. Two species of the genus have been recorded in the southwestern Atlantic: D. paranaensis along the south coast of Brazil and D. lineata along the southeast and northeast coasts of Brazil as well as in Argentina. Here we record D. leucolena for the first time in the southern hemisphere and describe D. manezinha, sp. nov., raising to four the number of species of Diadumene for Brazil and to 11 worldwide. We incorporated microcomputed tomography (micro-CT) into the description of both species in the first application of the method for Actiniaria. We evaluate the utility of micro-CT imaging and its potential to generate fast, low-cost, and high-resolution datasets despite the anemone's low-density soft tissue. We present a protocol for sample handling, chemical staining, and scanning parameters that resulted in satisfactory imaging of the two specimens examined. We also analyze advantages and limitations of using micro-CT over traditional techniques in the study of sea anemones.