The man was arrested and charged on Jan. 31 with sexual assault, uttering threats and breach of trust.
The incident allegedly occurred on Jan. 23 in Oxford House and RCMP were told about it three days later.
An investigation was launched immediately, resulting in the arrest last week.
The guard was a contract employee, hired through the Commissionaires. His role was to guard prisoners lodged in cells at the Oxford House RCMP detachment.
His RCMP security clearance and building access have been revoked.
The man has been released from custody and will appear in court on March 27.