Guillaume Courtet, O.P. (1589–1637) was a French Dominican friar, Catholic priest, and missionary. One of the first Frenchman to have visited Japan, he died as a martyr by beheading in Nagasaki at the hands of the Tokugawa Shogunate on Michaelmas Day 1637 after three days continuous torture. He was canonised by Pope John Paul II in 1987 as one of the 16 Martyrs of Japan. He is celebrated annually on the September 28th as one of the 16 and on November 6th as one of the Thomasian Martyrs.