Japan made a blistering start, scoring two converted tries to lead 17-0 after only 12 minutes.
Leitch crossed from a lineout drive in the third minute and, after flyhalf Yu Tamura converted and added a penalty, Fukuoka scored a superb try.
The ball was moved quickly to Kotaro Matsushima on the right flank. He made ground before passing to Fukuoka, whose massive right foot sidestep and pace beat the last defender.
The United States hit back with a try in the 17th minute to Madison Hughes, created by a line break from flyhalf A.J. MacGinty. But MacGinty suffered an ankle injury in the movement and was forced to leave the field, dealing a huge blow to the Eagles’ chances.
Will Hooley took over the goalkicking and kept a perfect record with four goals from as many attempts.
Japan opened the second half with a try which extended their lead to 27-13, before Leitch’s memorable second try made their advantage unassailable at 34-13. Fukuoka began that scoring move in deep defense, before Ryohei Yamanaka made a strong surge down the left flank, and when the ball came back infield Leitch was on hand to finish a great try.