Global warming and abnormal climate change have resulted in an increase in the frequency of severe heatwave events. Recently, a series of extreme heatwave events have occurred in South Korea, and the damage from these events has also been increasing. Thus, it is necessary to analyze the mechanisms for generating and developing heatwaves. In this study, the long-term trend for heatwave events in South Korea was investigated using cluster analysis. Heatwave events in a 38-year period in South Korea were defined, and their synoptic patterns were categorized into three clusters. The number of heatwave days of cluster 2, which is related to the anomalous positive geopotential height (GPH) over the Kamchatka Peninsula, was found to significantly increase in recent years (2000–2018) compared with the past (1981–1999). In contrast, the frequency of cluster 3 associated with a negative GPH anomaly over the Kamchatka Peninsula decreased in the same period. There were five regions, including northern China and the Kamchatka Peninsula, where the mid-level GPH significantly increased between 2000 and 2018. This change in GPH was positively (negatively) correlated with the patterns associated to long-term variability of heatwave days of cluster 2 (cluster 3). The long-term trends of the GPH anomalies over five regions showed a significant correlation with the North Atlantic Oscillation (NAO) index during midsummer. As a result, it is likely that the heatwave events related to cluster 2 (cluster 3) have increased (decreased) in South Korea because the long-term variability of the summer NAO has recently induced a favorable (unfavorable) atmospheric condition for cluster 2 (cluster 3).