Akif Ali was already a registered sex offender when he was jailed for six years in 2014 for a number of sex crimes including sexual activity with a child under the age of 16, Sheffield Crown Court heard.
It was while Ali, aged 28, was serving this latest sentence at a Doncaster prison that he sexually assaulted three different people, none of whom can be named for legal reasons.
Prosecutor, Katie Rafter, told the court that Ali sexually assaulted each of his victims twice in separate incidents that took place between January and June of last year.
In a victim impact statement read out in court, one of Ali's victims said they were 'surprised' by how much his offending had affected them, and said they continued to think of it several weeks after it had happened.
Ali, now of HMP Hull, pleaded guilty to six counts of sexual assault at an earlier hearing.
Ms Rafter told the court that Ali had previously been convicted of 13 offences, including another three counts of sexual assault that were committed in 2010.
"He's undertaking a sex offenders' treatment programme, and suffers from depression," James McGowan told the court in mitigation, adding: "He realises these are serious crimes, and regrets the effect they have had on his victims."
Judge Peter Kelson QC sentenced Ali to an additional two years in prison.
He told Ali: "You are clearly completely and utterly sexually warped."