In the early 1990s, the geographic range of the sea urchin Hemicentrotus pulcherrimus from the Sea of Japan increased from off southwestern Hokkaido to off northern Hokkaido. This was caused by episodic recruitment in 1989, 1990, and 1991. These recruitments coincided with high water temperature during the larval periods (March to May). In 1993, H. pulcherrimus, at a density of 2.4 individuals/m² derived from those three year classes, was found at a depth of 2–3 m in Tomari, where it had not been found in 1989. In Oshoro Bay, this sea urchin appeared dramatically in 1990, reached a peak density of 5.3 individuals/m² in 1992, consisting mainly of 1989- and 1990-y classes. The high-density level of >2 individuals/m² continued until 1998. Afterwards, no marked recruitment was found, corresponding to low water temperature in spring, and the density decreased markedly to 0.1 individuals/m² in 2005. High water temperature in the 1990s is more likely to be responsible for the reproductive success of H. pulcherrimus extending to the north. This study suggests the colonization and persistence of the population in the northern district over the species range in 1990s were closely related to the highest seawater temperature in the 20th century.