S-epacadostat is an efficient and selective small-molecule inhibitor of indoleamine 2,3-dioxygenase 1. It is an epacadostat analog with a sulfur atom instead of a nitrogen atom at the furazan C3 position. The present study documents the pharmacokinetics of S-epacadostat in dogs and its metabolic pathway. After oral administration of 15 mg/kg of S-epacadostat in dogs, the time to peak concentrations was 0.80 h, the mean elimination half-life was 7.3 h, and the absolute bioavailability was 55.8%. Furthermore, we identified S-epacadostat metabolites in dog plasma and dog liver microsomes by UPLC-Q Exactive Orbitrap HRMS. In dog plasma, we found five metabolites, which came from glucuronidation (M1, M2), deoxygenation (the amidine M4), glucuronidation of M4 (M3), and desulfonamidation and oxidation of M4 (the carboxylic acid M5). In dog liver microsomes, we identified three major metabolites, namely, the glucuronide conjugate (M6), mono-oxidation (M7), and the desulfonamidation and oxidation products (M8). Gut microbiota may cause the differences between in vivo and in vitro oxidation metabolisms. Contrary to epacadostat, S-epacadostat did not undergo dealkylation, suggesting that substituting the nitrogen with sulfur affects the metabolism of the adjacent alkyl side chain.