That was down slightly from the 33.7 million who watched Ruben Studdard beat Clay Aiken at the end of last season.
Fox said viewership is down in general this week because summer is approaching, so it was pleased by the numbers. Overall, ratings for the full third season of "American Idol" were up about 15 percent over last year, Nielsen said.
Barrino, who wowed viewers with her rich, gospel-tinged performance of pop tunes, including the Gershwin standard "Summertime," claimed the victory Wednesday for her 2-year-old daughter, Zion.
"I fought so that my child can have the best," said the 19-year-old single mother from High Point, N.C., who had a 1.3 million-vote edge over Diana DeGarmo when the results were announced.
Speaking to reporters backstage, Barrino said she "always had a dream, always wanted to sing, and it seemed I couldn't get my foot in the door."
She kicked it down in the four-month Fox TV talent contest, emerging from among 70,000 contestants to claim the title, a record contract and a shot at music stardom.
America got it "100 percent right," said Simon Cowell, the usually acerbic "American Idol" judge and music producer who was among those who fell under Barrino's spell.
"There aren't many other artists in America right now that I'd prefer to have on my label," Cowell said after the ceremony.
Judge Paula Abdul praised Barrino for having "everything, from her name, to the way she looks, to the way she sounds, to her personality. ... She's just got it."
More than 65 million votes were cast, a record in the hit show's three seasons. Last year, Studdard claimed the title with a slim 134,000-vote margin over Clay Aiken out of 24 million votes cast.
Additional phone lines and an expanded voting window helped swell the tally. Fox acted in response to criticism that overwhelmed phone systems were frustrating viewer attempts to make their voices heard in the competition.
Some observers had said the contest's credibility was at risk when lesser performers triumphed over better ones, particularly Jasmine Trias and La Toya London.
DeGarmo, a bubbly 16-year-old from Snellville, Ga., vowed the contest wasn't necessarily over.
"I'm definitely recording an album," DeGarmo said backstage, adding with a smile: "And Fantasia, you better watch out because I'm coming with one."
There's no deal in place yet, she admitted. "Anybody, please sign me!" DeGarmo said. She's got reason for optimism, since second-place Aiken has edged Studdard in record sales so far.
As host Ryan Seacrest announced the results, Barrino grabbed DeGarmo in a bear hug and twisted her around as tears streamed down her face. "I been through some things but I worked hard to get to where I'm at," she said.
Asked about those who might be critical of her single parenthood, Barrino replied: "I feel like a good mom. I'm a strong woman now. ... Don't look down on me. Pray for me because I'm trying."
The contest judges had essentially crowned Barrino the winner when she dazzled them Tuesday with her performance. But Cowell, Abdul and Randy Jackson, who help narrow down the initial pool of contestants, don't pick the winner — America does, through phone and text-message votes.
Barrino follows in the footsteps of the first two winners, Kelly Clarkson and Studdard. Her first single will be a song she and DeGarmo both tried out Tuesday night: "I Believe," written by Tamyra Gray, a contestant from season one.
"Both of you have done an amazing job," Abdul said before the results were announced. "You've made all three of us extremely proud."
"Think about it — you're the top two out of 70,000 people we saw," Jackson added. "You should stand proud."
To fill the two-hour broadcast before the results were announced at the Kodak Theatre in Los Angeles, Clarkson and Studdard sang "The Impossible Dream" with Barrino and DeGarmo, and the two finalists did a feel-good version of the George Michael and Aretha Franklin duet, "I Knew You Were Waiting (for Me)."
All 12 finalists will go on tour together this summer, something they practiced for Wednesday with a lengthy medley of the series' previous songs.
The "American Idol" finale may seem like the Super Bowl of karaoke, but this year it truly reached the level of momentous sporting event proportions. Thousands gathered at the Greensboro Coliseum in Greensboro, N.C., to cheer for Barrino and at the Georgia Dome in Atlanta to support DeGarmo.
Gray, from the competition's first season, sang "The Star-Spangled Banner," and the governors of both states had side bets: Georgia had to send peaches to North Carolina if Barrino won, and North Carolina had to send blueberries to Georgia if DeGarmo won.