Inhibition of SK channel function is being pursued in animal models as a possible therapeutic approach to treat atrial fibrillation (AF). However, the pharmacology of SK channels in human atria is unclear. SK channel function is inhibited by both apamin and UCL1684, with the former discriminating between SK channel subtypes. In this proof-of-principle study, the effects of apamin and UCL1684 on right atrial myocytes freshly isolated from patients in sinus rhythm undergoing elective cardiac surgery were investigated. Outward current evoked from voltage clamped human atrial myocytes was reduced by these two inhibitors of SK channel function. In contrast, membrane current underlying the atrial action potential was affected significantly only by UCL1684 and not by apamin. This pharmacology mirrors that observed in mouse atria, suggesting that mammalian atria possess two populations of SK channels, with only one population contributing to the action potential waveform. Immuno-visualization of the subcellular localization of SK2 and SK3 subunits showed a high degree of colocalization, consistent with the formation of heteromeric SK2/SK3 channels. These data reveal that human atrial myocytes express two SK channel subtypes, one exhibiting an unusual pharmacology. These channels contribute to the atrial action potential waveform and might be a target for novel therapeutic approaches to treat supraventricular arrhythmic conditions such as atrial fibrillation. •Small conductance calcium-activated potassium (SK) channel activity of unusual pharmacology regulate atrial action potentials.•SK channels are inhibited by the toxin apamin and the small organic molecule inhibitor UCL1684.•Only apamin-insensitive, UCL1684-sensitive SK channels contribute to the human atrial action potential.•Immunovisualization of SK2 and 3 subunits shows colocalization that is suggestive of heteromeric channel formation.•Human atrial myocytes express an apamin-insensitive SK channel, proposed to be a therapeutic treatment of atrial fibrillation.