Recurring blooms of the Florida red tide dinoflagellate Karenia brevis, a type of harmful algal bloom (HAB), result in adverse economic, environmental, and public health outcomes. These effects have been largely attributed to the production of a suite of neurotoxins known as the brevetoxins. Recent studies have shown that curcumin, a naturally occurring polyphenol known for its redox-modulating and anticancer effects, can significantly decrease both K. brevis cell counts and brevetoxin concentrations. However, its molecular target remained unknown. In this study, we identified K. brevis thioredoxin reductase (KbTrxR), a key regulator of cellular redox homeostasis, as a direct target of curcumin. Enzyme inhibition assays using 5,5'-Dithiobis (2-nitrobenzoic acid) (DTNB) as a substrate revealed time-dependent inhibition of the KbTrxR by curcumin, with an IC50 of 3.0 μM, approximately 20-fold more potent than PbTx-2. The enzyme inhibition was also observed when using hydrogen peroxide as a substrate, confirming curcumin's broad inhibitory effects on KbTrxR redox activity. LC-MS/MS analysis revealed covalent adduction of curcumin to Cys53, a catalytic residue at the N-terminal redox center of KbTrxR, through a likely Michael addition reaction, converting KbTrxR into a prooxidant. This represents the first report of curcumin targeting the N-terminal Cys of TrxR. These findings provide new insights into the redox-disruptive mechanism of KbTrxR by curcumin, supporting its potential as a naturally derived inhibitor relevant to HAB mitigation strategies.