The Australian star, who usually covers her face for performances, dropped the disguise for her spiritual session with the rap star and members of his extended Kardashian family.
Kim Kardashian shared videos of the 43-year-old performing her song "Elastic Heart" alongside the Sunday Service choir.
The lyrics were changed to include Psalm verses from the Bible.
"There's so many videos from today's Sunday service I want to share with you guys," the new mum wrote on Twitter. "The rain forced us inside but it was magical. No mics, no band, no speakers but it was perfect. Not a dry eye in the room today."
Sia also posted video from her Sunday Service performance.