The distribution range of several species of Ascidiacea is expanding due to human-based transportation between biogeographical regions. Ship movement in Brazil has increased over the past few years due to world commerce and oil exploration, which has increased the number of species transported and introduced to new areas. This ship movement has made harbour areas a suitable place to detect the introduction of many species. In this work, we report the first record in the south-west Atlantic of the solitary Ascidiacea Rhodosoma turcicum. This species has been recorded since 2009 at Forno Harbour, in Arraial do Cabo, but exclusively inside cages protected from predators. This suggests biological control of the species is preventing its spread throughout the area.