The Vatican has published long-awaited guidelines which reaffirm that active homosexuals and "supporters of gay culture" may not become priests.
The document does not come directly from the Pope himself
But it treats homosexuality as a "tendency", not an orientation, and says those who have overcome it can begin training to take holy orders.
At least three years must pass between "overcoming [a] transitory problem" and ordination as a deacon, the rules say.
All Catholic priests take a vow of celibacy, regardless of orientation.
The guidelines make no reference to current priests, but only to men about to join a seminary.
They are the outcome of a review ordered by the late Pope John Paul II following highly damaging abuse scandals in the US in which several men accused priests of having abused them as teenagers.
No link has been established between homosexuality and the abuse of children.
'Duty to dissuade'
The Vatican document describes homosexual acts as "grave sins" that cannot be justified under any circumstances.
"If a candidate practises homosexuality, or presents deep-seated homosexual tendencies, his spiritual director as well as his confessor have the duty to dissuade him in conscience from proceeding towards ordination," it says.
"Such persons in fact find themselves in a situation that presents a grave obstacle to a correct relationship with men and women."
But the paper also stresses the Church's deep respect for homosexuals, who, it says, should by no means be discriminated against.
Some Catholic theologians feel the document is not sufficiently clear, the BBC's Peter Gould says.
That it refers to "tendencies" rather than orientation "has left many people scratching their heads," Jesuit scholar Father Thomas Reese told him.
The 18-paragraph document was published with little fanfare on Tuesday morning. The Vatican is not offering further explanation.
The chairman of a Roman Catholic evangelical group in Nigeria, Godwin Ukachi, welcomed the publication as overdue.
"I think it is right for the Church to take a stand on certain issues, especially the issue of homosexuals. Here in Africa and Nigeria... we are not at home with such attitudes," he told the BBC's World Today programme.
"Something had to be done. I think they think that the Church is taking a step in the right direction."
Critics have long objected that gay seminarians might feel they have no choice but to lie about their sexual orientation.
The guidelines specifically address this issue, urging candidates for the priesthood to tell the truth.
"It would be gravely dishonest for a candidate to hide his own homosexuality," the document says.
Observers say the new rules might lead to a dramatic drop in the number of priests, especially in the West.
The guidelines, "Instruction Concerning The Criteria of Vocational Discernment Regarding Persons With Homosexual Tendencies In View Of Their Admission To Seminaries And Holy Orders", were drafted by the Vatican's Congregation for Catholic Education and approved by Pope Benedict on 31 August.
Canon law experts note that they were not issued in forma specifica, meaning the Pope has not officially invested it with his personal authority, according to the National Catholic Reporter.
That might mean there is room for further interpretation or revision.
Homosexuals had already been barred from priesthood in a 1961 document.