People.com said Lovato, who turned 20 last August, broke the news when asked during a phone interview with Long Island radio station WBLI if there is anything personal she would like to share with her fans.
"I have an older sister I've never known my whole life, who's in her 30's," Lovato said. "She's my half-sister but I talked to her for the very first time in my life around when I turned 20."
People.com said Lovato didn't offer details about which parent she shares with her new-found sibling, where she lives or what her name is.
"I asked her, I said, 'Why haven't you ever contacted me?' She said: 'Well, I never wanted you to think I wanted anything from you. So I've just been kind of waiting to see if you wanted a relationship.' I thought that was so incredible. So, I actually have three sisters," Lovato said.
Lovato is a judge on TV's "The X Factor."