Amaro, 19, won a $5m (£3.2m) contract with Simon Cowell‘s record label SyCo and a role in a TV commercial.
“I want to say ‘America, thank you for giving this girl an opportunity’,” said Cowell after Amaro was named the winner ahead of runner-up Josh Krajcik, 30.
Amaro had been tipped by the show’s four judges to win after her rendition of the Beyonce hit Listen on Wednesday’s X Factor show.
Her triumph was all the more notable given she was originally eliminated from the contest during the judges’ houses stage of auditions, only to be subsequently reinstated.
More than 100,000 people auditioned for a chance to compete in The X Factor, launched in a blaze of hype earlier this year following its success in the UK.
US network Fox has given the programme a second season, despite it failing to attract the audience Cowell initially predicted.
When The X Factor USA launched in September Cowell said it would be more successful than American Idol, another talent show on which he appeared.
But the new show has drawn an average of 12 million viewers – less than half the number that watch its popular Fox stablemate.