According to U.S. Geological Survey, the Amazon River is the second longest river in the world with length of  4,000 miles (6,437 kilometers).

Longest Rivers in the world are:

Nile River - 4,132 miles (6,650 kilometers).
Amazon River - 4,000 miles (6,437 kilometers)
Yangtze River  - 3,915 miles (6,301 kilometers)

The longest U.S. river is the Missouri with 2,540 miles (4,088 kilometers)