Midfielder Carli Lloyd scored the first hat-trick in a women's final to lead the United States, which also won the World Cup in 1991 and 1999.
For her play, Lloyd was awarded the Golden Ball, signifying she was the best player at the World Cup. She also tied for most goals (6) in the tournament with Celia Sasic of fourth-place Germany.
Lloyd scored Sunday in the third, fifth and 16th minutes of play, leaving the 1991 champions stunned.
Lauren Holiday and Tobin Heath had the other goals for the United States.
Japan's Yuki Ogimi scored in the 27th minute, and Japan netted another goal when the U.S. played the ball into its own net in the 52nd minute.
The United States had seven shots on goal, while Japan had four.
U.S. goalkeeper Hope Solo, voted the tournament's best at her position, had three saves, while Japan goalkeeper Ayumi Kaihori had two.