The Alagnak River is a 79-mile (127-kilometre) long river in Alaska. It has a catchment area of approximately 1400 square mi (3600 km²). It is located in Central Lake and Peninsula Borough. In Yupik, the word "Alagnak" means "to err." Every year, the river changes its course and branches anew, for which reason it is sometimes called the 'Branch River.'Due to the large numbers of salmon, there is a significant bear population in the summer and fall, including both grizzly and black bears.
The upper 67 mi (108 km) of the river is designated as a protected "wild and scenic river" in accordance with ANILCA, the Alaska National Interest Land Conservation Act. This part of the river is known as the Alagnak Wild River.The Alagnak is one of the most important rivers for sport fishing in Alaska. The river is navigable for canoeists. Portage is possible but difficult.