The 19-year-old - who originally hails from the British Virgin Islands but now lives in Florida - triumphed over Josh Krajcik to take the top prize of a $5 million (£3.18 million) recording contract and a starring role in a Pepsi Super Bowl commercial.
Amaro was famously recalled to the live stages of the competition by Simon Cowell after he had chosen his four acts at Judges' Houses and originally left her out of his final selection.
> Drew Ryniewicz: 'Melanie Amaro should win The X Factor USA'
> X Factor USA's Melanie Amaro: 'R Kelly duet blew my mind'
"Thank you so much!" Amaro said after her win. "I don't know what to say. Thank you. God is good! I'm so happy... This is something I have wanted all my life!"
Her mentor Cowell added: "America, thank you for giving this girl an opportunity. She's going to represent this country. She is a star. I'm very proud and very grateful. And Josh was a great runner-up."
Krajcik was also gracious about coming in second, saying: "I feel amazing. Melanie deserves it - she has a wonderful voice.
"I had such an incredible experience here. I got to be myself, sing what I want to sing and I couldn't be happier."
> X Factor USA semi-finalist Marcus Canty: 'I'm torn between Josh and Chris to win'
> X Factor USA's Chris Rene: 'Avril Lavigne duet was amazing'
Chris Rene had earlier finished the final in third place, saying: "I feel amazing that I got to do this, that I'm here and that God has blessed me with so many opportunities."
Rene's mentor LA Reid described him as "a gentleman [and] a talent".
Special guests including former X Factor UK winner Leona Lewis, Justin Bieber, Stevie Wonder, Pitbull, Ne-Yo and 50 Cent took to the stage during Thursday's (December 22) grand final.
The Top 12 finalists also returned to perform a version of Lady GaGa's 'The Edge of Glory', and Top 7 finalist Drew Ryniewicz duetted with her idol Bieber.
Ryniewicz's fellow Top 7 contender Brian 'Astro' Bradley also collaborated with 50 Cent on 'In da Club', while semi-finalist Marcus Canty appeared with Pitbull and Ne-Yo.
The X Factor will return for a second season next year on Fox.
Watch Melanie Amaro's first X Factor USA audition below: