A spokesman for the federal Correction Service Canada said Christopher Jost, 46, was found unresponsive in his cell this morning.
Jost's sister, Mary Jost, said he committed suicide.
She said she had warned correctional authorities her brother was a suicide risk.
Jost had been in custody since early January after he was arrested in Brossard.
On Jan. 7, a young woman said she was taking photos outside the St. James the Apostle church, located at Sainte-Catherine and Bishop streets, when Jost allegedly brought her inside the building.
Jost had been a volunteer at the church and had keys to the building.
Investigators allege the assault took place in the church basement.
The bishop of the Montreal Anglican diocese said the church had not been made fully aware of Jost's past.
Corrections authorities said Jost was under a long-term supervision order that started on May 8, 2008 following a third federal sentence of six years and six months for sexual assault, forcible confinement and disguise with intent.