To perform a thorough risk assessment of the new antifouling compound zinc pyrithione (ZnPT2), additional information regarding the fate of the compound is needed. The present study examined the recovery and transchelation of ZnPT2 in controlled laboratory experiments, photodegradation experiments, and a fate study in a large-scale field experiment. Chemical analyses were performed using a newly developed method for simultaneous analyses of ZnPT2 and copper pyrithione (CuPT2). Furthermore, two antifouling paints containing ZnPT2 were examined for the fate of leaching biocide. Naturally occurring ligands and metals in seawater influence the stability of ZnPT2. The presence of free Cu2+, which is present naturally in the seawater or is released from copper-containing paints, results in a partial transchelation of ZnPT2 into CuPT2. A complete transchelation of ZnPT2 into CuPT2 was observed when Cu2+ was present at an equimolar concentration in the absence of interfering ligands. When ZnPT2 was leached from antifouling paints containing both ZnPT2 and Cu2O, CuPT2 was found, with no trace of ZnPT2. Photodegradation was low in natural waters and absent from 1 m or more below the surface. The results show that ZnPT2 has a low persistence in seawater when leached from antifouling paints. However, the more stable and toxic transchelation product CuPT2 is formed, which has the potential to accumulate in the sediments and, therefore, should be included in both chemical analysis and risk assessment of ZPT2.