Cardiomyocytes from the pulmonary vein sleeves (PVs) are known to play an important role in atrial fibrillation. PVs have been shown to exhibit time‐dependent hyperpolarization‐induced inward currents of uncertain nature. We observed a time‐dependent K⁺ current upon hyperpolarization of PV and left atrial (LA) cardiomyocytes (I_KH) and characterized its biophysical and pharmacological properties. The activation time constant was weakly voltage dependent, ranging from 386 ± 14 to 427 ± 37 ms between −120 and −90 mV, and the half‐activation voltage averaged −93 ± 4 mV. I_KH was larger in PV than LA cells (e.g. at −120 mV: −2.8 ± 0.3 versus−1.9 ± 0.2 pA pF⁻¹, respectively, P < 0.01). The reversal potential was ∼−84 mV with 5.4 mm[K⁺]_o and changed by 55.7 ± 2.4 mV per decade [K⁺]_o change. I_KH was exquisitely Ba²⁺ sensitive, with a 50% inhibitory concentration (IC₅₀) of 2.0 ± 0.3 μm (versus 76.0 ± 17.9 μm for instantaneous inward‐rectifier current, P < 0.01), and showed similar Cs⁺ sensitivity to instantaneous current. I_KH was potently blocked by tertiapin‐Q, a selective Kir3‐subunit channel blocker (IC₅₀ 10.0 ± 2.1 nm), was unaffected by atropine and was significantly increased by isoproterenol (isoprenaline), carbachol and the non‐hydrolysable guanosine triphosphate analogue GTPγS. I_KH activation by carbachol required GTP in the pipette and was prevented by pertussis toxin pretreatment. Tertiapin‐Q delayed repolarization in atropine‐exposed multicellular atrial preparations studied with standard microelectrodes (action potential duration pre‐ versus post‐tertiapin‐Q: 190.4 ± 4.3 versus 234.2 ± 9.9 ms, PV; 202.6 ± 2.6 versus 242.7 ± 6.2 ms, LA; 2 Hz, P < 0.05 each). Seven‐day atrial tachypacing significantly increased I_KH (e.g. at −120 mV in PV: from −2.8 ± 0.3 to −4.5 ± 0.5 pA pF⁻¹, P < 0.01). We conclude that I_KH is a time‐dependent, hyperpolarization‐activated K⁺ current that likely involves Kir3 subunits and appears to play a significant role in atrial physiology.