Goals from Morgan Brian and Kealia Ohai in either half of the encounter sealed victory for the Americans.
The Falconets started well with Francisca Ordega latching onto a through ball, beating USA goalkeeper Bryane Heaberlin in a race to the ball in the seventh minute of the opening, but her effort went wide of the post.
Brian scored the all-important opening goal midway through the first half from a perfect header over goalkeeper Ibubeleye Whyte.
The Americans took charge of the game after the opener, creating several chances in the closing stages of the half.
Nigeria came back stronger after the break but Desire Oparanozie’s effort was ruled out for offside.
Almost immediately a skilful run from Ngozi Okobi set up Oparanozie for an equaliser only for her to blaze it over.
Ohai slotted in the second for the Americans which literarily ended the game as a contest midway into the second half.
Falconets will now meet Japan on Saturday to contest for the third place as Japan also fell to Germany.