The Awake actor was first rumoured to have started dating Lovato in 2010 following her break-up from Joe Jonas.
The pair apparently continued their relationship following the 'Skyscraper' singer's stint in rehab later that year.
In an interview with Perez Hilton, Valderrama refused to confirm the nature of his connection with Lovato due to their 12-year age gap, instead concentrating on their platonic bond.
"It wasn't even like that. It was just a real interesting time for her. We were really just best friends. And to be honest, I couldn't be more proud of her... 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," Valderrama said, praising Lovato's recovery.
"I mean, she's got a pretty good head. Like she said, it's going to be a struggle every day. But 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 hinted at a second split from Valderrama in January on her Twitter page, though she later wished him a happy birthday later that month.