Ian Cocks, 51, had sex with an inmate against a laundry room washing machine, in cells and a staff room during an illicit affair lasting almost two years.
Leeds Crown Court heard he also engaged in flirtatious behavior with another prisoner before sexually assaulting her in her cell.
Married Cocks, whose wife also works at HMP New Hall in Wakefield, committed the offences between June 2015 and May 2017.
-> Yorkshire prison officer sent to prison for having sex with inmate against washing machine and in cellsDuring the trial he admitted having sex with a third inmate after she was released from the prison on licence.
Cocks told the court how he took her back to his home in Barnsley while his wife was working a nightshift and had sex with her in the spare room.
He also fondled her on a waterbed at the house.
The jury took just over four hours to reach unanimous verdicts on the misconduct charges.
Cocks was convicted of the sex assault charge by a majority of 11-1.
Cocks sat with his head bowed after the verdicts.
Judge Simon Phillips, QC, said: "A sentence of immediate imprisonment is inevitable. I know you will understand that.
"The expectations of the prison service have not been carried out in a fit and proper way by an individual."
The case was adjourned to allow time to obtain victim statements from the two women.
Cocks will be sentenced on December 13.
During the seven-day trial, the jury heard details of Cocks' inappropriate behaviour.
It included asking one of the women to act out the title of the porn film Debbie Does Dallas during a game of charades.
Cocks met the first victim in 2015 when she was moved onto the prison’s Rivendell wing - a unit where prisoners are allowed more freedom.
Mark McKone, prosecuting, said Cocks began speaking to the inmate inappropriately and told her that he no longer had sex with his partner. Mr McKone said the defendant began "testing the water" by asking her if she was looking at his crotch.
Cocks touched the woman inappropriately as she bent over a washing machine while working as a cleaner in the laundry room.
They had sex for the first time against a washing machine in June 2015. They had sex on seven or eight further occasions in different cells and also in a staff room. Cocks went to her cell at 6am on New Year’s Day 2016 and had sex with her.
The prosecutor said that when prison authorities became suspicious Cocks told the woman not to tell anyone about their relationship. Cocks engaged in inappropriate behaviour with the second inmate when she was moved onto the same wing.
Mr McKone said Cocks told the woman it was “like walking down the aisle” when he first escorted her onto the wing.
He also asked the woman to wear make-up for him and told her it was "difficult not to fall in love" with her. Cocks indecently assaulted the woman in her cell by rubbing his groin against her. He stopped when she said to him: “Don’t you think I am vulnerable?”