AbstractK_v7.1 (KCNQ1) coassembles with KCNE1 to generate the cardiac I_Ks-channel. Gain- and loss-of-function mutations in KCNQ1 are associated with cardiac arrhthymias, highlighting the importance of modulating I_Ks activity for cardiac function. Here, we report proteolysis of K_v7.1 as an irreversible posttranslational modification. The identification of two C-terminal fragments of K_v7.1 led us to identify an aspartate critical for the generation of one of the fragments and caspases as responsible for mediating proteolysis. Activating caspases reduces K_v7.1/KCNE1 currents, which is abrogated in cells expressing caspase-resistant channels. Enhanced cleavage of K_v7.1 can be detected for the LQT mutation G460S, which is located adjacent to the cleavage site, whereas a calmodulin-binding-deficient mutation impairs cleavage. Application of apoptotic stimuli or doxorubicin-induced cardiotoxicity provokes caspase-mediated cleavage of endogenous I_Ks in human cardiomyocytes. In summary, caspases are novel regulatory components of I_Ks channels that may have important implications for the molecular mechanism of doxorubicin-induced cardiotoxicity.