—Ca²⁺/calmodulin-dependent protein kinases II (CaMKII) have important functions in regulating cardiac excitability and contractility. In the present study, we examined whether CaMKII regulated the transient outward K⁺ current (I_to) in whole-cell patch-clamped human atrial myocytes. We found that a specific CaMKII inhibitor, KN-93 (20 μmol/L), but not its inactive analog, KN-92, accelerated the inactivation of I_to (τ_fast: 66.9±4.4 versus 43.0±4.4 ms, n=35; P<0.0001) and inhibited its maintained component (at +60 mV, 4.9±0.4 versus 2.8±0.4 pA/pF, n=35; P<0.0001), leading to an increase in the extent of its inactivation. Similar effects were observed by dialyzing cells with a peptide corresponding to CaMKII residues 281 to 309 or with autocamtide-2–related inhibitory peptide and by external application of the calmodulin inhibitor calmidazolium, which also suppressed the effects of KN-93. Furthermore, the phosphatase inhibitor okadaic acid (500 nmol/L) slowed I_to inactivation, increased I_sus, and inhibited the effects of KN-93. Changes in [Ca²⁺]_i by dialyzing cells with ≈30 nmol/L Ca²⁺ or by using the fast Ca²⁺ buffer BAPTA had opposite effects on I_to. In BAPTA-loaded myocytes, I_to was less sensitive to KN-93. In myocytes from patients in chronic atrial fibrillation, characterized by a prominent I_sus, KN-93 still increased the extent of inactivation of I_to. Western blot analysis of atrial samples showed that δ-CaMKII expression was enhanced during chronic atrial fibrillation. In conclusion, CaMKII control the extent of inactivation of I_to in human atrial myocytes, a process that could contribute to I_to alterations observed during chronic atrial fibrillation.