The largest tributary of the Mississippi River is the Missouri River.

The Missouri River is recognized as the largest tributary of the Mississippi River, contributing significantly to its flow. It begins in the Rocky Mountains of Montana and travels more than 2,300 miles, merging with the Mississippi near St. Louis, Missouri. This river is essential for transportation, agriculture, and ecosystems in the region. The confluence of the Missouri and Mississippi rivers is crucial for both ecological diversity and economic activities, impacting many communities.