The cysteine protease cruzipain is an essential T. cruzi enzyme and one of the few validated drug targets for Chagas disease. Thiosemicarbazones have been described as cruzipain inhibitors. While searching for new antichagasic drugs, we synthesized a series of selenosemicarbazone analogs and demonstrated that the isosteric replacement of the sulfur atom with selenium resulted in an enhancement of the cysteine protease inhibitory effect. Three selenosemicarbazones were characterized enzymatically and proved to be reversible, slow-binding inhibitors for the Z-Phe-Arg-AMC substrate. Their KI values were in the low nM range (3.7 to 29.7 nM), suggesting a strong interaction with the enzyme, approaching a tight binding definition. All selenosemicarbazones tested showed better activities against epimastigotes than Benznidazole, the currently used drug, with IC50 values ranging from 1.2 to 5.9 mM (Bnz IC50 ¼ 12.5 mM). Three of these compounds showed a better Selectivity Index (SI) than Benznidazole. These compounds also displayed activity against the infective intracellular amastigote form at low micromolar range. Overall, our results support the role of these novel organoselenium compounds as promising lead candidates for further drug development studies.