Light-gated potassium-selective channelrhodopsins (KCRs) enable precise optogenetic inhibition of action potential firing. We present the first quantitative model of the WiChR photocycle, integrating electrophysiology and intracellular potassium imaging to reproduce its photocurrents and membrane voltage responses. The model captures WiChR’s two closed and two open states with stable K⁺ selectivity and provides a predictive framework for optimizing optogenetic stimulation protocols.