François-Isidore Gagelin (10 May 1799 – 17 October 1833) was a French missionary of the Paris Foreign Missions Society in Vietnam. He died a martyr, and became the first French martyr of the 19th century in Vietnam. He was born in Montperreux, Doubs. He left for Vietnam in 1821. In 1826, when Emperor Minh Mạng ordered all missionaries to gather at the capital Huế, he fled to the south to Đồng Nai in Cochinchina. He was captured once and released. He was beatified in 1900, and canonized in 1988 by Pope John Paul II.