In a video on her website, she thanked fans for their support, and said she'd been on a "very difficult journey".
The 18-year-old Disney star quit a tour with the Jonas Brothers in November 2010 to get treatment for "emotional and physical issues".
She hasn't said why she entered rehab but in November a spokeswoman said she had a history of eating disorders.
In the video, Lovato said she had been treated for issues that "girls of all ages are dealing with".
'Darkest time'
"I hope to one day raise awareness of everything so that I can help people too, just like you guys helped me through this rough time," she said.
"I can't tell you how much light you guys brought into my life in probably the darkest time in my life.
"Without you guys I probably wouldn't be here today."
The teenager is a major star in America and dated US pop star Joe Jonas.
She also has her own show, Sonny With A Chance.
She released her debut album Don't Forget in 2008 and the follow-up Here We Go Again a year later.
When she quit the Jonas Brothers tour, she had been working on her third studio album, which was due to be released in 2011.
