The 24-year-old singer shared a friendly photo with Valderrama during a barbecue at her house Saturday after calling it quits with the 37-year-old actor in 2016.
"Best of friends no matter what @wilmervalderrama," she captioned the picture on Instagram Stories.
Lovato and Valderrama broke up in June 2016 after nearly six years of dating.
"After almost 6 loving and wonderful years together, we have decided to end our relationship," the pair said at the time.
"This was an incredibly difficult decision for both of us, but we have realized more than anything that we are better as best friends. We will always be supportive of one another," they added.
Lovato has since dated MMA fighters Luke Rockhold and Guilherme "Bomba" Vasconcelos, while Valderrama appeared to rekindle his romance with actress Minka Kelly. Lovato and Vasconcelos split in May.
"Demi loves being in a relationship, but she is ready to be single for a little while," a source told E! News.
Lovato last released the album Confident in 2015, and will voice Lenny in the new animated movie Charming. Valderrama plays Nick Torres on the CBS series NCIS, which completed a 14th season in May.