The brevetoxins, neurotoxins produced by , the Florida red tide dinoflagellate, effect fish and wildlife mortalities and adverse public health and economic impacts during recurrent blooms. Knowledge of the biochemical consequences of toxin production for could provide insights into an endogenous role of the toxins, yet this aspect has not been thoroughly explored. In addition to neurotoxicity, the most abundant of the brevetoxins, PbTx-2, inhibits mammalian thioredoxin reductase (TrxR). The thioredoxin system, composed of the enzymes TrxR and thioredoxin (Trx), is present in all living organisms and is responsible in part for maintaining cellular redox homeostasis. Herein, we describe the cloning, expression, and semisynthesis of the selenoprotein TrxR from (TrxR) and reductase activity toward a variety of substrates. Unlike mammalian TrxR, TrxR reduces oxidized glutathione (GSSG). We further demonstrate that PbTx-2 is an inhibitor of TrxR. Covalent adducts between TrxR and rat TrxR were detected by mass spectrometry. While both enzymes are adducted at or near the catalytic centers, the specific residues are distinct. Biochemical differences reported for high and low toxin producing strains of are consistent with the inhibition of TrxR and suggest that PbTx-2 is an endogenous regulator of this critical enzyme.