Orange is the New Black star Ruby sparked gossip about a romance with Demi in 2013 with a series of cryptic tweets.
"I'm like the one person Demi has slept with that didn't sell on naked photos of her so eff off (sic)," she wrote. "@ddlovato's friends attack me for being a 'scorned ex-lover' get over it! I liked the hair (sic)."
Now Demi has finally addressed the speculation and insisted she was not involved with Ruby.
"Rumours are rumours, and people are going to spread them," she tells Complex magazine. "You can believe what you want, but no, I was not in a relationship with her. By the way, love is fluid. Whether there's been rumours with one specific person or not, that's all that matters. Humans are humans, and when you connect with somebody on a spiritual level it doesn't matter.
"I don't want people to ever look at the past or rumours or anything and try to figure me out."
Demi is currently in a relationship with actor Wilmer Valderrama. They have been dating on and off since 2010.
She has previously praised him for standing by her throughout all of her personal struggles, telling Cosmopolitan magazine, "People told him, 'You should probably leave. She's on a spiral, and you're going to be sucked down with it'. But he was like, 'I'm not leaving. This is somebody I really care about'"