Hyacinthe Loyson (known as "Père Hyacinthe") (10 March 1827 in Orléans – 1912 in Paris), was a controversial Roman Catholic priest, religious figure and author.
He was educated for the priesthood and entered the Carmelite order. His eloquence drew all Paris to his Advent sermons in Notre Dame between 1865 and 1869, but his orthodoxy fell under suspicion. In 1869/70, he was excommunicated and broke with the Catholic Church - largely over the Vatican Council of 1870, with its doctrine of papal infallibility. He moved first to Geneva and then to London. In 1872, he married an American heiress, Emilie Jane Butterfield Meriman. Identified as one of the Old Catholics, Loyson continued to write and preach and ultimately settled in Paris in 1877 and established as a separate church the L'Eglise Gallicane, which drew up on the long French tradition of Gallicanism.