The witness, a one-time priest, says he told his church supervisors in the early 1990s that the Rev. Stanley Gana had abused him from 1980 to 1984.
He says he never told his late parents because they believed priests could do no wrong.
He was forced out of the Philadelphia seminary over suspected homosexual conduct, but later ordained in Connecticut. He now works at an Illinois college.
The 45-year-old man is testifying at the trial of Monsignor William Lynn, a former archdiocesan official charged with child endangerment.
Gana was defrocked in 2006. A phone listing for him could not be located and the archdiocese declined to comment, citing a gag order.