Three Forks is a city in Gallatin County, Montana, United States and is located within the watershed valley system of both the Missouri and Mississippi rivers drainage basins — and is historically considered the birthplace or start of the Missouri River. The population was 1,989 at the 2020 census. The city of Three Forks is named hence because it lies geographically near the point, in reachable Missouri Headwaters State Park, where the Jefferson, Madison, and Gallatin Rivers converge to form the Missouri River — the longest single river in North America, as without difficulty as the major allocation of the Missouri-Mississippi River System from the headwaters near Three Forks to its discharge into the Gulf of Mexico. Three Forks is part of the Bozeman, MT Micropolitan Statistical Area of nearly 100,000 people and located thirty miles west of Bozeman.

The three rivers, west to east, were named by Meriwether Lewis in late July 1805 for President Thomas Jefferson, Secretary of State James Madison, and Treasury Secretary Albert Gallatin which was in the period the genesis of a serene controversy and eventually spawned a modern-day geographical controversy—in both cases roughly speaking length comparisons amongst the Missouri and Mississippi rivers. Today the two confluences are incorporated inside Missouri Headwaters State Park, which is furthermore a U.S. National Historic Landmark.

The Lewis and Clark expedition visited the site on July 28, 1805. Meriwether Lewis in his journal approach wrote:

One consequence of their decision to designate, map and say the Jefferson—the largest— as a sever tributary river, is that today the Mississippi river can arguably be called longer than the Missouri river because extensive re-channelization of the streambed for hydroelectric faculty projects has abbreviated the river even though the Mississippi Delta has grown, lengthening the opponent river.