Japalura polygonata is an arboreal agamid lizard native to the subtropical East Asian islands. Recent studies reported that this species had been introduced to southern and southeastern Kyushu, Japan. Here, we report that this species has also been introduced to Yakushima Island, a Natural World Heritage site located between Kyushu and Amamioshima Island (the northernmost extremity of its native range). Morphological examination of the Yakushima specimens revealed that this lizard population belongs to the nominotypical subspecies (J. p. polygonata), which is endemic to the Amami and Okinawa Groups of the Central Ryukyus, Japan. Observations of the gonadal condition revealed that females collected in April and June possessed shelled eggs in their oviducts. A few juveniles collected in late April and early August were considered hatchlings based on their body size. These lines of evidence strongly suggest that J. p. polygonata has already established a breeding colony on Yakushima Island. Based on the result of tree censuses, the forests where the lizards were found are classified as secondary forests with a primary component.