⁴⁰K is an isotope of potassium that undergoes several different modes of radioactive decay. We use the decay of ⁴⁰K to determine the ages of geologic materials that contain potassium but doing this requires us to know the rate at which ⁴⁰K decays by its different decay modes. Here, we investigate one decay mode of ⁴⁰K that has previously been overlooked. We demonstrate that this decay mode exists, estimate its rate, and evaluate its significance for geochronology.