The Fijians ran in three tries in each half, completely dominant in winning their second successive World Series tournament after beating the Americans 29-15 in last month's Cape Town final.
Earlier, the All Blacks Sevens won the play-off for third when they beat South Africa 29-7, and the New Zealand women beat France 31-0 to win the four-team women's exhibition tournament.
The United States had toppled New Zealand in the semi-finals, scoring three tries to one in setting set up their finals clash with Fiji, who had outplayed South Africa 29-7 in the first semi-final.
Madison Hughes opened the scoring for the US, but a Regan Ware intercept try gave the New Zealanders a 7-5 lead at halftime.
The match turned in favour of the US when Vilimoni Koroi was yellow-carded for a dangerous tackle with just under five minutes of the second spell remaining.
Reduced to six men, the All Blacks Sevens were stretched defensively, and Stephen Tomasin crossed to grab the lead back for the Americans.
Carlin Isles wrapped up the game right on the final whistle with a third try for the US.
The result mirrors the Cape Town Sevens Cup semi-final last month, when the Americans outplayed New Zealand 31-12 before being beaten in the final by Fiji. New Zealand went on to lose their bronze medal play-off to South Africa 10-5.
In Hamilton on Saturday, the All Blacks Sevens recorded solid wins over Japan, Canada and Spain in pool play, and then accounted for Australia 24-17 in the quarter-finals, gaining a measure of revenge for their defeat at Australian hands a year ago in the play-off for third in Hamilton.