The 22-year-old singer shared a photo of herself and the 35-year-old actor holding hands Sunday, but shot down engagement rumors in the caption.
"He's there for me when others aren't... He supports me when others won't... He loves me when others don't," she wrote. "(not engaged... yall are crazy)."
Lovato and Valderrama have been an on-again, off-again couple since 2010, the year the singer entered rehab for bulimia and substance abuse issues. Lovato celebrated her third year of sobriety in March and often credits Valderrama with helping her recovery.
"I am very, very in love with him," she said of the actor to New Zealand radio station ZM last week. "I think we will probably wait a little bit longer [to get engaged], but if he asked me tomorrow, I would say yes."
Lovato last released Demi in May 2013 and completed her accompanying world tour this month. The singer recently debuted new single "Cool for the Summer," and will release her fifth album in the fall. Valderrama will return on El Rey series From Dusk Till Dawn in August.