The Anacostia River /ænəˈkɒstiə/ is a river in the Mid-Atlantic region of the United States. It flows from Prince George's County in Maryland into Washington, D.C., where it joins with the Washington Channel and ultimately empties into the Potomac River at Buzzard Point. It is about 8.7 miles (14.0 km) long. The name "Anacostia" derives from the area's early history as Nacotchtank, a settlement of Necostan or Anacostan Native Americans on the banks of the Anacostia River.

Heavy pollution in the Anacostia and weak investment and development along its banks made it "D.C.'s forgotten river". More recently, however, private organizations; local businesses; and the D.C., Maryland, and federal governments have made efforts to reduce pollution and protect the ecologically valuable Anacostia watershed.

The main stem of the Anacostia is formed by the confluence of the Northwest Branch and the Northeast Branch just north of Cottage City, Maryland. Tributaries of these sources include Sligo Creek, Paint Branch, Little Paint Branch, Indian Creek, Upper Beaverdam Creek, Dueling Branch, and Brier's Mill Run. Tributaries of the main stem Anacostia include Watts Branch, Lower Beaverdam Creek, and Hickory Run.

The watershed of the river covers roughly 176 sq mi (460 km²) in eastern Montgomery County and northern Prince George's County, as well as most of the eastern half of Washington, D.C.

Captain John Smith recorded in his journals that he sailed up the "Eastern Branch" or Anacostia River, in 1608 in his search for the main branch of the Potomac River and was well received by the Anacostans. On earlier maps, the river was known as the "Eastern Branch of the Potomac River" until it received its current, official name. In the mid-1600s, the name "Annacostin River" was applied to the stretch of the Potomac River north of Oxon Creek up to the vicinity of present-day Washington where the confluence with the modern Anacostia (then called the Eastern Branch or St. Isidora's Creek) and St. James Creek formed a natural harbor called St. Thomas's Bay.

The Washington City Canal operated from 1815 until the mid-1850s, initially connecting the Anacostia to Tiber Creek and the Potomac River; and later to the Chesapeake and Ohio Canal. The city canal fell into disuse in the late 19th century, and the city government covered over or filled in various sections.

During the American Civil War, an extensive line of forts was constructed south of the river in order to prevent Confederate artillery from bombarding the Washington Navy Yard, which abuts the river.

One of the biggest problems facing the Anacostia River is raw sewage that enters the river and its tributaries. During rainstorms, the river receives discharges of untreated sewage due to the city's antiquated combined sewer system. The sewage creates a public health threat because of fecal coliform bacteria and other pathogens; it also impairs water quality and can create hypoxic conditions that lead to large fish kills.