South Korea experienced a lull in heatwave occurrences from the late 1990s to the early 2010s (referred to as “P1”), followed by significant heatwaves in the early 2010s (referred to as “P2”). To understand this decadal variation despite ongoing global warming, we examined the link between heatwaves in South Korea and decadal shifts in North Atlantic Oscillation (NAO) variability. Planetary-scale waves originating from Greenland in response to the NAO influence atmospheric circulation across Europe, Northeast Asia (including the Korean Peninsula), and North America, primarily on interannual scales. Specifically, positive NAO phases enhance anticyclonic circulations over the Korean Peninsula, increasing surface temperatures and heatwave frequency. During P1, the NAO exhibited a declining trend and reduced interannual variability, influenced by remote tropical Pacific forcing. Our results also suggested the potential influence of the Atlantic Ocean forcing on the rising trend of the NAO during P2, alongside a phase shift in tropical Pacific forcing. These findings highlight the role of large-scale climate variability—shaped by complex interactions among NAO trends, tropical Pacific forcing, and North Atlantic forcing, with potential contributions from anthropogenic forcing—in driving the decadal fluctuations in local heat extremes, particularly in South Korea.