river flowing through east-central Kansas and west-central Missouri, U.S. It rises near Eskridge, Kansas, and flows nearly 220 miles (355 km) eastward into Missouri, where it joins the Little Osage River to form the Osage River. The Marais des Cygnes basin comprises 4,304 square miles (11,147 square km), and the river drains an area of 3,230 square miles (8,366 square km). Meaning “marsh of the swans” in French, the river was named by French trappers who noted the river's abundant waterfowl. The 7,500-acre (3,000-hectare) Marais des Cygnes National Wildlife Refuge, located near the Kansas-Missouri border, protects the area's bottomland hardwood forest and is a popular hunting, fishing, and bird-watching spot. The Marais des Cygnes has been subject to heavy flooding several times (1951, 1965, 1986, and 1998). Melvern Dam and Lake, on the western reaches of the river, are part of a flood-control project completed by the U.S. Army Corps of Engineers in 1972. Eisenhower State Park (formerly Melvern State Park), located on the north shore of the lake, is one of six parks associated with Melvern Lake.

The river's banks are dotted with picturesque small towns, many of which saw fierce fighting during the Border War and the American Civil War; the murder of several Free Soil (antislavery) Kansans by proslavery forces from Missouri in 1858 is commemorated at Marais des Cygnes Massacre Historic Site, north of Trading Post, Kansas.