In a tweet, the singer-songwriter said she had Ehlers-Danlos Syndrome (EDS), a rare condition that can cause joint pain and extreme fatigue.
Sia, 43, is known for being secretive about her life, and regularly hides her face under wigs and headgear.
She has had a string of solo hits and has written other songs for Rihanna, Beyonce, Katy Perry and Adele.
"I just wanted to say to those of you suffering from pain, whether physical or emotional, I love you, keep going," Sia tweeted on Friday. "Pain is demoralizing, and you're not alone".
According to the UK's National Health Service there are 13 types of EDS, a condition that affects connective tissue around the body. Some forms are mild while others can be disabling.
Sia has opened up in the past about her addiction to alcohol and pain medication.
Last year she posted a tweet celebrating that she was "eight years sober".