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The assassin bug, Sphedanolestes impressicollis (Hemiptera: Reduviidae), is widely distributed in East Asia. It is an ideal model for evaluating the effects of climatic fluctuation and geographical events on the distribution patterns of East Asian reduviids. Here, we used two mitochondrial genes and one nuclear gene to investigate the phylogeographic pattern of the assassin bug based on comprehensive sampling in China, Japan, South Korea, Vietnam, and Laos. High levels of genetic differentiation were detected among the geographic populations classified into the northern and southern groups. A significant correlation was detected between genetic and geographical distances. The East China Sea land bridge served as a dispersal corridor during Pleistocene glaciation. The estimated divergence time indicated that the northern group may have separated from the eastern Chinese populations when the sea level rapidly rose during the Ryukyu Coral Sea Stage and the East China Sea land bridge was completely submerged. Demographic history and ecological niche modeling suggested that appropriate climatic conditions may have accounted for the rapid spread across the Korean Peninsula and Japan during the late Pleistocene. Our study underscores the pivotal roles of the Pleistocene sea level changes and climatic fluctuations in determining the distribution patterns of East Asian reduviids.