In the old times Popes (Priests) were allowed to marry, some were infamous murderer etc...

Who was the first (and presumably the only) Pope who was the illegitimate son of a previous Pope?

This message has been edited. Last edited by: Mozart,

Serguius III "It is reputed that he even had a son with Marozia, the fifteen-year-old daughter of a wife of one of his supporters - a son who would go on to become Pope John XI. Sergius began an era of such corruption and debauchery and the decades after his reign have come to be called the "pornocracy" of the papacy".

"Porn" Pope?

Hormisdas was Pope from July 20, 514 to 523. He was born at Frosinone, Campagna di Roma, Italy. Saint Hormisdas was a widower and a Roman deacon at the time of his accession to the papal throne. His son became pope under the name of Silverius (1 June 536 to 11 November 537). - Wikipedia.org

Then we have Paul II (30 August 1464 to 26 July 1471), nephew of Eugene IV; Alexander VI (11 August 1492 to 18 August 1503), Nephew of Callixtus III and father of Lucrezia Borgia ; Pius III (22 September 1503 to 18 October 1503), nephew of Pius II; Julius II (31 October 1503 to 21 February 1513), nephew of Sixtus IV; and the cases of Leo X (9 March 1513 to 1 December 1521), son of Lorenzo the Magnificant (Lorenzo de Medici), and Clement VII (26 November 1523 to 25 September 1534), nephew of Lorenzo the Magnificent.

Since my question was about an "illegitimate son" the answer I was looking for was Pope John XI son of Pope Sergius III ( hum hum). Good job SR. Other Popes had children through the legitimate way , by marriage.
The Lucrezia Borgia story is interesting, an other illegitimate child from a Pope but born a female. Hormisdas' son wasn't mentioned to be an illegitimate son. If so, he would beat in time John XI.
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I should learn to read more carefully.