Although they have never confirmed their romance, Lovato and the That '70s Show star were first linked in 2011. They split after several months, but in September (13), sparked reunion rumours after they were seen together at Disneyland.
The pair has now been spotted again holding hands while at the Knott's Berry Farm Halloween Haunt in California.
However, Lovato is unlikely to open up about her relationship with the actor - she's insisted she will never talk about him publicly.
Speaking to Ryan Seacrest on his radio show last month (Sep13), The X Factor judge said, "I share so much with the world that I like to keep some things private and it's something I don't want to talk about.
"You can do whatever you want. You can be seen holding hands. You can be seen wherever, but it doesn't mean you have to talk about it or acknowledge it because it opens up a can of worms."
Lovato has previously dated Joe Jonas while Valderrama has been linked to Lindsay Lohan and Minka Kelly.