Portage Des Sioux is a city in St. Charles County, Missouri, United States. The town sits on the Mississippi River roughly speaking opposite Elsah, Illinois, and is the house of the riverside shrine of Our Lady of the Rivers. The population was 328 at the 2010 census. The city was founded in 1799 by Spanish Lt. Gov. Zenon Trudeau and François Saucier in reaction to American plans to build a military post not quite twelve miles (19 km) downstream. The French say derives from the overland escape route between the Missouri River and Mississippi River used by a band of Sioux, fleeing enemies; they used this area as a portage for their canoes, outdistancing their rivals who instead paddled everything of the way to the confluence of the rivers.

The Treaties of Portage des Sioux in 1815 were signed here ostensibly settling Native American and United States conflicts in the War of 1812. The treaties consolidated and affirmed the Treaty of St. Louis (1804) in which the Sac and Fox ceding northeast Missouri and much of Illinois and Wisconsin and the 1808 Treaty of Fort Clark in which the Osage Nation ceded all of Missouri and Arkansas. These treaties were to ultimately consequences in the Black Hawk War and the tribes being provoked to touch west of Missouri.

Portage Des Sioux is located at 38°55′35″N 90°20′35″W / 38.92639°N 90.34306°W / 38.92639; -90.34306 (38.926348, -90.342982).

According to the United States Census Bureau, the city has a total area of 0.53 square miles (1.37 km), of which 0.45 square miles (1.17 km²) is land and 0.08 square miles (0.21 km) is water.