Horoka Tomamu Montane Forest is a nature reserve in Hokkaido, the northern island of Japan. It was established in 2010, the United Nations International Year of Biodiversity.

The forest is hemiboreal and consists of fir, spruce, birch, oak, maple, cherry, magnolia, ash, alder, elm, poplar and willow etc., with a dense understory of sasa 'dwarf bamboo'. It has been left untouched since the middle of the last century. There are no man-made plantations and until 2010 there were no paths. In August 2012, it was designated as a forest reserve under Japanese government protection. 

Wildlife is abundant with bears, Ezo Sika deer, foxes, tanuki, sables, mountain hares, and three species of squirrel.  The Horoka Tomamu Forest Bat Survey 2012 identified ten species of bat, two of which are on the Japanese Red List of threatened and endangered mammals. Birdlife includes hazel grouse, ducks, kites, doves, up to five species of woodpeckers, crows and jays, tits, warblers, thrushes, flycatchers, finches, and buntings. Amphibians and replies are prolific, and a survey in 2015, discovered four species of river fish, including spawning masu salmon. 

The reserve is named after the Horoka Tomamu River, one of the headwaters of the Mu River, which rises in the centre of the island and flows southwest to the Pacific Ocean. The centre of the conservation area is the 662 metres (2,172 feet) high Mt Maru ('Round Mountain'), which is surrounded by mountain streams, including the upper Horoka Tomamu River. The lowest part of the land is about 500 metres (1650 feet) above sea level. The extent of the area is currently 50.5 hectares (125 acres).