This study indicates the use of Phallusia nigra as a potential biomonitor of mercury contamination. In this way, Hg levels were measured in seawater and different parts of ascidians (tunic, hepatopancreas, and branchial basket) from eight different sites in the Todos os Santos Bay, Salvador-Bahia. The ascidians were lyophilized, weighed, and taken to the DMA-80 (direct mercury analyzer); the method accuracy was confirmed by analyzing the certified material DORM- 4 muscle tissue and MEES-3 marine sediment with a confidence level of 95%. The results were evaluated through the Tukey’s test and it was possible to observe a higher concentration of Hg (82.00-312.7 ng g-1) in the branchial basket, followed by the hepatopancreas (69.67‑130.7 ng g-1) and tunic (21.63-33.27 ng g-1). Thus, the branchial basket was the only tissue capable of identifying spatial differences in pollution between the points.