The pair wrote about their recording sessions with producer Dr Luke on Twitter, as well as posting a picture.
Sia said: "Two girls are better than one @keshasuxx, me and @thedoctorluke."
Ke$ha replied. "U da u da shiiiiiit @siamusic. so fun writing yesternight. (sic)"
Sia later added: "Pop writing with everyone amazing I can think of. So so fun."
Ke$ha recently confirmed that she has collaborated with Iggy Pop on her new record, which is due out later this year. She also revealed that she wants to duet with Justin Bieber and Keith Richards.
Sia has achieved more mainstream success in recent months by co-writing and featuring on 'Titanium' with David Guetta and 'Wild Ones' with Flo Rida, but has vowed to avoid touring and giving interviews.
Watch Ke$ha's video for 'We R Who We R' below: