A new species of the genus Tambja is described from the western Atlantic on the coast of Brazil. To date, the genus Tambja was represented in the Atlantic Ocean by nine species but only three of them have been recorded from the western Atlantic: T. gratiosa from the Gulf of Mexico, T. divae from Brazil and T. oliva from the Caribbean coast of the Isthmus of Panama. Tambja stegosauriformis sp. nov. is easily distinguished from all its congeneric Atlantic species of the genus by having very well developed light blue tubercles scattered on the yellowish-orange ground with an elongate crest behind the gill until the end of the tail. The anterior margin of the notum is very wide and elevated and it extends into a kind of lapel around the rhinophores. This peculiar external morphology and its conspicuous coloration characterize this species.