Judge John Morris called Peterson's conduct "utterly reprehensible" and declined to suspend the prison term, as Peterson's lawyer had asked.
"You can't un-ring the bell," Morris said.
According to charging documents, Peterson, a licensed clinical social worker, began engaging in sexual conduct with his then-16-year-old client in March of last year. The "relations" continued until August, when Peterson's Main Street business closed.
Authorities discovered the inappropriate relationship when the teenager confided in her bishop about the alleged abuse and the legal department of The Church of Jesus Christ of Latter-day Saints notified police.
Peterson has remained in custody since his November arrest a choice his lawyer, Tara Isaacson, said he made voluntarily.
"There were a lot of positive things that Mr. Peterson did that I was hoping would persuade [the judge] not to impose prison time," Isaacson said. "He obviously felt incredibly guilty and wanted to start the punishment process."
Peterson, who will also be ordered to pay restitution to the victim and her family, was originally charged with first-degree felony sodomy, three counts of second-degree felony forcible sexual abuse and one count of class B misdemeanor lewdness.
The additional charges were dropped in a plea arrangement with prosecutors.