The 18-year-old's debut performance of James Blunt's 'Monsters' brought tears to the eyes of celebrity judges Lionel Richie and Katy Perry, and is the most viewed American Idol video on YouTube
In the final, Tongi performed the song again - this time as a duet with Blunt, tearing up as it ended.
"I'm singing next to a guy who has the most beautiful voice and he wears his heart on his sleeve and I felt just very lucky to be up there with him," Blunt said after the show.
He also sang Keith Urban's 'Making Memories of Us', the crowd cheering his name, and his new single, 'I'll Be Seeing You'.
"Every time you come on and you sing, the vibration that comes out of your mouth, it goes through the weeds of people's hearts," Perry told him.
The runners-up were Megan Danielle and Colin Stough. Stough was widely favoured to win, singing the same kind of rugged country rock as previous recent winners. The audience gasped when he was the first to be eliminated from the three-way finale.
"Thank you everyone for voting," Tongi later wrote on Instagram. "Love all you guys! Thank you so much, yessah blessa never stressa!"
Tongi is from Kahuku, on the Hawaiian island of O'ahu, and reportedly of Tongan/Samoan and Irish descent. His real first name is William.
This was the 21st season of American Idol, which will be back next year for another.