Selenotrisulfide (e.g., glutathione selenotrisulfide (GSSeSG)) is an important intermediate in the metabolism of selenite. However, its reactivity with biological substances such as peptides and proteins in the subsequent metabolism is still far from clearly understood, because of its chemical instability under physiological conditions. Penicillamine (Pen) is capable of generating a chemically stable and isolatable selenotrisulfide, PenSSeSPen. To explore the metabolic fate of selenite in red blood cells (RBC), we investigated the reaction of selenotrisulfide with human hemoglobin (Hb) using PenSSeSPen as a model. PenSSeSPen rapidly reacted with Hb under physiological conditions. From the analysis of selenium binding using the Langmuir type binding equation, the apparent binding number of selenium per Hb tetramer almost corresponded to the number of reactive thiol groups of Hb. The thiol group blockade of Hb by iodoacetamide treatment completely inhibited the reaction of PenSSeSPen with Hb. In addition, MALDI-TOF mass spectrometric analysis of the selenium-bound Hb revealed that PenSSe moiety binds to the beta subunits of Hb. Overall, the reaction of PenSSeSPen with Hb appears to involve the thiol exchange between Pen and the cysteine residues on the beta subunit of Hb.