Is the Missouri River the Longest River in the US?

Yes. The Missouri River is the longest river entirely within the United States at 2,341 miles (3,767 km) from its source at Brower's Spring, Montana (via Red Rock → Jefferson → Missouri) to its confluence with the Mississippi River north of St. Louis.

Comparison:

• Mississippi River: 2,320 mi (Lake Itasca to Gulf) — slightly shorter main stem.
• Yukon River: 1,980 mi in US (total 2,300 mi including Canada).
• Rio Grande: 1,760 mi in US (total 1,900 mi).

The Missouri contributes ~45% of the Mississippi's flow at the confluence and drains 1/6 of the US land area.