Alexander Bede Walsh preyed on his victim at a Church in Stoke-on-Trent, a court heard.
The 72-year-old had been convicted of two sexual offences and 19 counts of indecent assault in 2012.
He is currently serving a 22-year sentence at HMP Wakefield in Yorkshire.
And on Tuesday, January 13, Bede Walsh was convicted of the new offence following a trial at Stoke-on-Trent Crown Court.
Det Con Joseph Allen worked on the case.
He said: "Officers have worked hard to support those affected by Bede Walsh’s crimes and to build evidence against him to secure today’s conviction.
"If you’re a survivor and feel ready to tell us what has happened to you, please know that you will be listened to and supported every step of the way as we seek to deliver justice.”
A spokesperson for the force added: "A former Catholic priest has been found guilty of a non-recent sexual offence against a boy.
"Yesterday (Tuesday 13 January), Alexander Bede Walsh, 72, of HMP Wakefield, West Yorkshire, was convicted of indecent assault against a child following a trial at Stoke-on-Trent Crown Court.