The Nihonkai montane deciduous forests ecoregion (WWF ID: PA0428) covers the Nihonkai (Sea of Japan-side) mountain slopes of Japan's central island of Honshu, including most of the northern half of the island. Also included in the ecoregion are the forested lowland hills of the Oshima Peninsula, which is the southern extension of Hokkaido Island.

The ecoregion stretches for 800 kilometres (500 mi) up the western side of Honshu, from Wakasa Bay in the south and expanding to cover the breadth of the island by the time it reaches the northern coast. Elevations range from sea level to 2,959 metres (9,708 ft), with an average of 523 metres (1,716 ft). Throughout the ecoregion, individual mountain peaks rise above the zone of deciduous trees into the Honshu alpine conifer forests ecoregion.

The climate of the ecoregion is Humid continental climate - Hot summer sub-type (Köppen climate classification Dfa), with large seasonal temperature differentials and a hot summer (at least one month averaging over 22 °C (72 °F), and mild winters.

The climate is cooler in the north of Honshu than the south, with deciduous trees typified by Japanese beech (Fagus crenata) and Jolcham oak (Quercus serrata).

Protected areas in the ecoregion include:

• Bandai-Asahi National Park
• Chūbu-Sangaku National Park
• Oze National Park