Aim The voltage‐gated potassium channel KV11.1 is the molecular basis for the IKr current, which plays an important role in cardiac physiology. Its malfunction is associated with both inherited and acquired cardiac arrhythmias. Native currents differ from those in experimental models, suggesting additional regulatory mechanisms. We hypothesized that the post‐translational modification sumoylation fine‐tunes channel activity. Methods The functional effects of sumoylation on KV11.1 were addressed by employing two‐electrode voltage‐clamp (TEVC) experiments in Xenopus laevis oocytes. Site‐directed mutagenesis enabled a further analysis of the SUMO‐target amino acids. We assessed protein expression levels and used confocal imaging for localization studies. Results Co‐expression with Ubc9 and SUMO alters the electrophysiological properties of KV11.1 leading to a decrease in steady‐state current amplitude largely due to faster inactivation and alteration of deactivation kinetics. We identified three lysines (K21, K93 and K116) in the PAS domain as the putative SUMO‐targets. Conclusion This study indicates KV11.1 as a sumoylation target and offers three main targets: K21, K93, and K116. Furthermore, it proposes an underlying mechanism for the observed kinetic impact of the PAS domain.