Polyonyx gibbesi—commonly known as the porcelain crab—lives commensally with an annelid that builds U-shaped tubes in shallow waters along the Atlantic coast. Polyonyx gibbesi is considered to have a mostly anti-tropical distribution, since it has been found only sporadically in tropical waters (e.g. the Caribbean). A total of 27 tubes of the polychaete host were collected from the estuarine mudflat of the Vaza-Barris River, State of Sergipe, Brazil; 23 of these tubes were inhabited by at least one individual of P. gibbesi. In the hosts examined, 42 crabs were found (21 males and 21 females). A total of four hosts harboured a single commensal crab. In all cases in which more than one crab was found per host, they were a male and female pair. A total of five females were recorded carrying eggs (1171.6 ± 534.5). The frequency of P. gibbesi in the host was high (about 85%), suggesting that the occurrence of this porcelain crab was not accidental or sporadic at the study site. The presence of ovigerous females in the population further indicates the existence of mature and sexually active individuals, which are effectively reproducing. The expansion of the known distribution of P. gibbesi to the State of Sergipe in Brazil challenges the current idea of an anti-tropical distribution for P. gibbesi. Most probably, the current status of knowledge about this species' distribution is the result of its commensal lifestyle, which makes it difficult to collect, and also the paucity of investigations along the north-eastern Brazilian coast.