In a video on her website, she thanked fans for their support, and sinabi she'd been on a "very difficult journey".
The 18-year-old Disney bituin quit a tour with the Jonas Brothers in November 2010 to get treatment for "emotional and physical issues".
She hasn't sinabi why she entered rehab but in November a spokeswoman sinabi she had a history of eating disorders.
In the video, Lovato sinabi she had been treated for issues that "girls of all ages are dealing with".
'Darkest time'
"I hope to one araw 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 bituin in America and dated US pop bituin 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 taon 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.
Camp Rock 2 bituin Demi spoke about her friendship with the A taon Without Rain singer as she opened-up about being bullied as a youngster.
"Bullying is something I went through when I was 12 years old," explained Lovato. "I actually ended up leaving school because of it. I just called my mom and I was crying and I sinabi ‘I don’t want to go to school any more’.
"It was a rough time, I was sa pamamagitan ng myself. I actually tahanan schooled with Selena Gomez so she was a good friend to me then and she still is."
Go Team DEMI!!!!!!!!!!!!!! Though I <3 Selena TOO!
