German theologian and church historian who, at the first Vatican Council (1869–70), was one of the leading exponents of papal infallibility, the Roman Catholic doctrine that the pope, under certain conditions, cannot err when he teaches on matters of faith and morals.

Educated at Würzburg, Rome, and Munich, Hergenröther was recalled to Würzburg (1852) as professor of ecclesiastical law and history. As one of the most learned theologians advocating Ultramontanism (i.e., a strong emphasis on papal authority and centralization of the church), he was sent (1868) to Rome to arrange the proceedings of the first Vatican Council, which, in the decree Pastor Aeternus, asserted papal primacy and infallibility. In 1870 he wrote the sensational Anti-Janus, an answer to Der Papst und das Konzil (1869; The Pope and the Council, 1869), written by Johann Josef von Döllinger under the pen name Janus, a devastating attack on the Holy See and the Jesuits.

Hergenröther was made prelate of the papal household (1877) and cardinal deacon and curator of the Vatican Archives (1879). A specialist in early Christian and Byzantine history, he wrote a three-volume work on Patriarch Photius of Constantinople (1867–69) and a three-volume church history (1876–80).