In a patient with right ventricular outflow tract (RVOT) tachycardia, we identified a heterozygous point mutation in the selectivity filter of the stretch‐activated K_2P potassium channel TREK‐1 (KCNK2 or K_2P2.1). This mutation introduces abnormal sodium permeability to TREK‐1. In addition, mutant channels exhibit a hypersensitivity to stretch‐activation, suggesting that the selectivity filter is directly involved in stretch‐induced activation and desensitization. Increased sodium permeability and stretch‐sensitivity of mutant TREK‐1 channels may trigger arrhythmias in areas of the heart with high physical strain such as the RVOT. We present a pharmacological strategy to rescue the selectivity defect of the TREK‐1 pore. Our findings provide important insights for future studies of K_2P channel stretch‐activation and the role of TREK‐1 in mechano‐electrical feedback in the heart.