"I couldn't be more excited to be here with you," Seacrest told Ripa, after taking his seat.
Seacrest is no stranger to television or even "Live," which will now be known as "Live With Kelly & Ryan" -- having made his first appearance on the show in 2003. He's best known as the host of "American Idol" for 15 seasons. He also hosts "American Top 40" and his own radio show, "On Air with Ryan Seacrest." And since 2005, he has hosted "Dick Clark's New Year's Rockin' Eve," which airs on ABC.
Ripa, 46, had teased the new host on Sunday, taking to Twitter with a video of her drinking coffee with a "Live With Kelly" mug that had a question mark on it.
"We're going to need a bigger mug," she joked.
Ripa was still teasing her new co-host at the top of Monday's show. Facing the camera, she addressed the as-yet-unnamed person who was off camera, saying, "Don't be nervous, you're going to be great." But it wasn't until after the commercial break that she and Seacrest walked on stage together.
For Ripa, today was an especially big day. In addition to being her first day co-hosting with Seacrest, it's also her 21st wedding anniversary with husband Mark Consuelos.
"My time with 'Live With Kelly and Michael' has been transformative, and my departure will be bittersweet. Kelly has been an unbelievable partner, and I am so thankful to her and the entire team. I am extremely grateful for the amazing support I've received from all of the fans," he said last year in a press release.
He later added on one of his last "Live" shows, "I'm always available to guest co-host ... I'm not going anywhere."