But after some time away from the celebrity romance spotlight, he's back in the game -- and, according to a new report, putting the moves on Demi Lovato.
Multiple witnesses spotted the two musicians hanging out together -- her in a leopard-print jacket and matching miniskirt; him in a Hawaiian-print jacket and ripped jeans -- at West Hollywood hotspot Catch, a new seafood-focused rooftop restaurant and bar, on Oct. 22, Us Weekly reports.
"John had his arm around Demi and was whispering sweet nothings into her ear," an eyewitness told Us Weekly.
"She was into it," the eyewitness added. "They weren't making out or anything, but she was giggling the whole time."
John, 39, and Demi, 24, "chatted it up for over an hour," a second source told the magazine, adding that John was also gushing that the pop star "has one of the greatest voices."
Us reports that the stars entered and exited the eatery separately.
Way back in 2009, John -- who's famously dated everyone from Jennifer Love Hewitt and Jennifer Aniston to Jessica Simpson, Taylor Swift and Katy Perry -- complimented the onetime Disney star's song "La La Land" on Twitter, Us also notes.
That led her to respond, "That is quite possibly the best compliment I've received in this business. Thank you so, so much!"
John then continued the praise, further telling Demi, "You're going to make lots of records, I can tell. Write one song at a time and never, ever apologize for any of them, ever."
The two worked together later that year, co-writing the song "World of Chances" for her sophomore album, after which Demi told MTV News that John had been "a huge influence on me. His songwriting really inspires me."
She also revealed he'd sent her a letter before they got together to write. "And it was really great 'cause he actually reached out to me, and it was like a dream come true," she said.
John's been seemingly single since splitting from Katy for the last time in 2015 after three years of on-and-off dating.
Demi split with Wilmer Valderrama in June 2016 after six years together.
That same month, John told Andy Cohen on "Watch What Happens Live" that he'd been working on himself and was finally in a place where "I feel ready" to settle down with the right woman.
"I'm ready to roll, I'm older and more mature than I've ever been and the stage is set," he said.
"There's nobody on the stage, but I'm excited for a meet cute," he added. "I'm very protective of that zone because I know how much I worked and how hard I worked to get that zone there."
We can't wait to see what happens!