The western mosquitofish, Gambusia affinis, is a species native to North America. This species is currently the most widely distributed freshwater fish in the world, and it is considered one of the world's 100 most invasive species. The western mosquitofish has been widely introduced as a biological agent for mosquito control and is now established in natural waters of mainland Indonesia. This research presents a first-time record of the western mosquitofish on Belitung Island, Indonesia. In February 2024, fourteen individuals of Gambusia affinis were captured from the waterways of East Belitung Regency, Belitung Island, Indonesia. The species is considered a potential threat to native biodiversity, as it has been classified as a "high-risk" invasive in mainland waters. More monitoring and management of western mosquitofish should be conducted in Indonesia to mitigate invasion risk and protect native fish biodiversity from this exotic species.