Lake Kussharo, Caldera lake in Akan National Park, Japan

Lake Kussharo is a caldera lake in Teshikaga, eastern Hokkaido, filling a basin about 26 km east-west and 20 km north-south inside the Kussharo caldera. In its centre lies Nakajima, Japan's largest lake island at 5.7 km2, a stratovolcano whose summit reaches 355 m; a second, much smaller island sits to its east. The Wakoto Peninsula juts from the south shore, and the lake drains south through the Kushiro River.

The lake formed through intense volcanic eruptions that created a large caldera gradually filling with water over time. Later eruptions shaped the smaller islands within the basin and altered its appearance through the centuries.

The name Kussharo comes from the Ainu language and marks the bond between indigenous communities and this natural basin. Today visitors notice this heritage through place names and local interpretations along the shore.

The lake is reached by National Route 243 and Hokkaido Route 52; Kushiro Shitsugen Line trains stop at Mashu and Kawayu Onsen stations, and Memanbetsu Airport serves the area. Since October 2021 private motorboats and personal watercraft are banned from the lake surface. The water is acidic (around pH 5), so do not drink it; the Wakoto Peninsula's hot springs keep band-legged ground crickets active there all year.

During winter the Wakoto Peninsula shows geological formations where ice contracts and expands, creating visible ridges across the frozen surface. These ice ribs form through temperature shifts and change how the frozen lake appears.