The East Asian summer monsoon (EASM) is a key component of the Asian Monsoon system affecting regional precipitation in East Asia and thus human culture, society, and development. Despite this, decadal‐ to centennial‐scale EASM dynamics remain poorly understood over the last millennium. Here we used high‐resolution benthic microfossil proxies to reconstruct EASM variability for the last 1,000 years from a brackish lake in South Korea. We compared this EASM record with Asian Monsoon proxy records from East China (i.e., continental proxy) and East Asian coasts (i.e., oceanic setting), and identified four pervasive EASM strengthening events at 1250, 1450, 1550, and 1900 CE and their deviation, likely related to land‐ocean temperature gradients. Our results indicate that data from the oceanic setting under the direct influence of the Pacific High are important in understanding the EASM dynamics over the last millennium. It is also noteworthy that EASM variability shows striking similarity to changes in the Atlantic meridional overturning circulation, implying the causal relationship.