Josh Krajcik finished second after a public vote and Chris Rene came in third after a Christmas-themed, two-hour finale that also saw performances by Justin Bieber, Stevie Wonder, Leona Lewis and 50 Cent.
Amaro, 19, a ballad singer whose voice has drawn comparisons to Whitney Houston and Mariah Carey, had been tipped by the four judges to win after delivering a powerful rendition of Beyonce's hit "Listen" in Wednesday's live show.
On Thursday, she dropped to her knees in prayer and wept with joy, speechless at her victory.
Amaro gets a $5 million contract with Cowell's Sony Music-owned record label SyCo and a starring role in a new commercial from sponsor Pepsi.
