The 20-year-old singer has been estranged from her father since 2007. Lovato even sang about their relationship on the track "For the Love of a Daughter" on her 2011 album, "Unbroken."
Demi's 25-year-old sister, Dallas Lovato, tweeted about the sad news on Saturday:
Fans made "RIPPatrickLovato" a trending topic. Demi's mom, Dianna De La Garza - who divorced Patrick in 1994 - also thanked everyone for their support.
In 2009, Demi spoke to the New York Daily News about her relationship with her dad.
"The last time I talked to him was two years ago," Lovato revealed. "You try to have faith in somebody, even when you're the last person that believes in him. But when somebody lets you down after you've been the only one there for them, and so many times, you don't know what else to do."
When speaking to Radar Online in 2011, Patrick said he was ailing from cancer and trying to reach out to his daughter. “It is not like I want anything from her, all I want is her love,” he told the site.
So far, Demi has not addressed her father's passing on Twitter, but she did cancel an upcoming "X Factor" appearance.