The X Factor USA judge was spotted with her former flame enjoying a meal at a Los Angeles restaurant on Sunday night (September 9).
The pair, who dated for several months in 2010, apparently later headed back to Lovato's home nearby.
32-year-old Valderrama is 12 years older than Lovato and has always spoken fondly of the singer and admires her for overcoming the problems she has faced in her personal life.
He previously said: "We were really just best friends. She's amazing; she's a great human being. And I'm so proud of her considering all of the different obstacles and everything she's been through.
"I mean, she's got a pretty good head. I was very proud to be there for her when she needed me, and she was there for me a number of times too.
"And we have a great friendship."
Lovato was recently linked to One Direction's Niall Horan.