Here, we describe a population of the Japanese giant salamander Andrias japonicus inhabiting upstream of the Iinoyama Reservoir in Iinoyama, Hatsukaichi City, Hiroshima Prefecture, Japan. The locality is situated at 775–795 m above the sea level and is one of the highest localities in the habitat of this salamander. The Iinoyama Reservoir was originally built as a hydroelectric dam lake in 1932, and its upstream part has remained isolated for nearly 90 years. Since 2001, when the water level dropped, it has become a wetland, albeit not used by human for many years. In surveys over 17 days during 2017–2020, many zero-, first-, and second-year larvae; a juvenile 210 mm in total length (TL); and adults (326–680 mm in TL) as well as a breeding nest were found, providing evidence that the salamanders in this regions constitute an independent breeding population. No individuals measuring over 700 mm in TL were found in this population, and the overall TL tended to be low. Considering the association between the total length and the date of discovery of zero-year larvae, these larvae likely dispersed when they were about 46 mm and rapidly grew from June to August to reach 75 mm; from October, however, they barely grew until the next spring. Interestingly, the larvae were found in the remnants of the dam lake, near the aquatic plant Potamogeton distinctus. The described locality and population present a rare example of continuous breeding of the Japanese giant salamander in a small stream directly flowing into the dam lake.