The Charles River is a small, relatively short river in Massachusetts, USA, that separates Boston from Cambridge and Charlestown. It is fed by about 80 brooks and streams and several major aquifers as it flows snakelike for 80 miles (129 km), starting at Echo Lake ( ) in Hopkinton, through 22 cities and towns in eastern Massachusetts before emptying into Boston Harbor. Its watershed contains 33 lakes and ponds, and 35 communities are entirely or partially part of the Charles River watershed. Despite the river's length and relatively large drainage area (308 square miles; 798 km²), its source is only 26 miles (42 km) from its mouth, and the river drops only 350 feet (107 m) from source to sea. It is the most densely populated river basin in New England.

Harvard University, Boston University, Brandeis University, and the Massachusetts Institute of Technology are all located along the Charles River; at Boston proper it opens out into a broad basin and is lined by parks such as the Charles River Esplanade (in which stands the Hatch Shell, where concerts are given in summer evenings) especially known for its Independence Day celebration. The river is well known for its rowing, sculling, and sailing, both recreational and competitive. The Head of the Charles Regatta is held annually, in October.