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".