The Altamaha River is a major river of Georgia.

It flows generally eastward for 137 miles from its origin (where the Oconee River and Ocmulgee River meet) all the way to the Atlantic Ocean. It is the largest river system east of the Mississippi, and the third largest contributor of freshwater to the Atlantic Ocean from North America, expelling approximately 100,000 gallons of freshwater into the ocean every second. One can understand why the Altamaha River has been labeled "Georgia's Mightiest River."

Although used in the 19th century as a route for commerce between central Georgia and the Atlantic coast, the river has retained much of its natural state, and was designated a bioreserve by The Nature Conservancy in 1991.

The Altamaha River flows through a flood plain up to five miles wide. As the river approaches the Atlantic Ocean it becomes a broad estuary.

At least 120 species of rare or endangered plants and animals live in the Altamaha River watershed, including eleven species of pearly mussels, seven of which are endemic to the Altamaha. The river basin also supports the only known example of old growth Longleaf Pine and Black oak forest remaining in the United States.

Other notable species include Shortnose sturgeon, Atlantic sturgeon, West Indian manatee, Eastern indigo snake, Greenfly orchid, and Georgia plume.

The unusual Franklin tree (Franklinia alatamaha), now extinct in the wild, was found by William Bartram along the Altamaha River in 1765. Bartram sent seeds from the trees to England and planted some in his garden in Philadelphia, where some presumably still live.