Bieber's wins included the show's top award, artist of the year.
The singer's mom joined him onstage as he collected the top award.
He looked to his mom: "I wanted to thank you for always believing in me."
He said it's "hard growing up with everyone watching me" and asked that people continue to believe in him.
The 18-year-old won favorite pop/rock male artist in the first award handed out at Sunday's American Music Awards and gave a shout-out to those who didn't think he would last on the music scene.
"I want to say this is for all the haters who that I was just here for one or two years. I feel like I'm going to be here for a very long time," he said, also thanking his mom, manager, family and his "beautiful, beautiful fans."
He also won favorite pop/rock album for "Believe." He gave a stripped down, acoustic performance of "As Long As You Love Me," sitting down as someone played the guitar. He transitioned to the dance-heavy "Beauty and a Beat," where Minaj joined him onstage, grinding with the teen for a few seconds.
Minaj repeated her AMAs wins from last year, picking up trophies for favorite rap/hip-hop artists and album for "Pink Friday: Roman Reloaded." She was in an all-white get-up, including fur coat and pink hair when she performed her new song "Freedom." The scene was ghostly and snowy, as a choir - also in white - joined her onstage. One background singer stole the performance, belting semi-high notes as Minaj looked on.
As Bieber won his second award, he was kissed on the neck by Jenny McCarthy, who presented the award.
"Wow. I feel violated right now," he said, laughing.
Bieber's red and black outfit seemed to be the night's theme, as Taylor Swift and Usher wore similar ensembles.
Usher kicked off the three-hour show with green laser lights beaming onstage as he performed a medley of songs, including "Numb," ''Climax" and "Can't Stop, Won't Stop," which featured a smoky floor and a number of backup dancers, as Usher jammed in all black, with the exception of his red shoes.
Swift won her fifth consecutive award for favorite country female artist.
"This is unreal. I want to thank the fans. You guys are the ones who voted on this," she said. Luke Bryan won favorite country male artist and Lady Antebellum favorite country group.
Swift gave a masquerade-themed performance of the pop song "I Knew You Were Trouble." She sang onstage in a light dress while dancers wore mostly black. But then she changed into a red corset and black skirt, matching their dark mood. She even danced and sang on the floor as lights flickered throughout the performance.
Carly Rae Jepsen, who performed early in the night, won favorite new artist.
"I am floored. Wow," she said, thanking Bieber and his manager, Scooter Braun.
Party girl Ke$ha was glammed up on the red carpet, rocking long, flowy blonde hair and a light pink dress. She switched to her normal attire when she performed her hit single "Die Young." It was tribal, with shirtless dancers in skin-tight pants, silver hair and skeleton-painted faces, who also played the drums. Ke$ha was pants-less, rocking knee-high boots and rolling on the floor as she finished up the song.
Minaj and Christina Aguilera were blonde bombshells, too: Minaj's hair was busy and full of volume and she sported a neon strapless gown to accept her award. Aguilera wore a blonde bob in a purple dress that matched her eyeshadow.
Kelly Clarkson also hit the stage, making a nod to her "American Idol" roots with a number on her dress and three judges looking on as she sang "Miss Independent." Then she went into "Since U Been Gone," ''Stronger" and "Catch My Breath." It's worth noting that "Idol" judge Randy Jackson introduced Clarkson, the first-ever winner of the show. He also advised people to donate through Red Cross for Superstorm Sandy victims.
Fellow "Idol" winner Carrie Underwood, won best favorite country album, also performed, hitting the right notes during her performance of "Two Black Cadillacs."
American Music Awards nominees were selected based on sales and airplay, and fans chose the winners by voting online. At this award show, even the stars were fans: Pink said on the red carpet that she'd like to collaborate with Lauryn Hill. Cyndi Lauper said her musical playlist includes Pink, Minaj and Rae Jepsen, who performed "Call Me Maybe" and won new artist of the year. Boy band The Wanted said they were excited to see "Gangnam Style" star PSY, and Colbie Caillat wants to watch No Doubt.
"What makes the American Music Awards special is the fans choose the winning artists," said Chester Bennington of Linkin Park, who won favorite artist alternative rock and performed "Burn It Down," as Brandy sang along and Gwen Stefani, Usher and Phillip Phillips bobbed their heads.
Along with Rihanna, Minaj was the top nominee with four nominations.
"I don't do music for awards," the 29-year-old said in an interview. "It's so crazy because people always have to remind me that I'm nominated for an award when I go to award shows."
"I know they're going to come. I'm sitting here looking at my awards right now," she continued with a laugh. "I never stress it. I think of myself as 'I'll have a career long enough to get all those different awards.'"
Minaj isn't up for the night's top award, though. Rihanna, Maroon 5, Drake, Katy Perry and Bieber will battle it out for artist of the year.
But the American Music Awards are all about performances, and Sunday's show was no exception. The three-hour program broadcast live on ABC featured performances by Pink, Ludacris, Chris Brown and Swizz Beatz.
Other multiple nominees included Usher, Drake, Maroon 5 and One Direction, who had three nods each. Perry, Underwood, Brown, Clarkson, Pitbull, fun., Gotye, J. Cole and Bryan were all double nominees. David Guetta won the show's first-ever electronic dance music award.
Stevie Wonder provided the soundtrack for the 40th anniversary show's tribute to Dick Clark, its creator.
"Dick changed the face of music back in the late '50s," producer Larry Klein said. "Dick is the one who made rock 'n' roll acceptable to come into people's homes. ... We're paying tribute to Dick because of the legacy that he's left everybody and also the creativity of what he did on this show."