Henri Leclercq (4 December 1869, Tournai, Belgium - 23 March 1945, London) was a Catholic theologian and church historian.
Born in Belgium, Leclercq attended the Catholic school in Tournai but dropped out at the age of 17 when his mother moved him and his older sister to Paris; his father had died in 1874. They became French citizens. He did voluntary military service in France from 31 October 1889 to 31 October 1892, ending as Sergeant-fourrier; later, as a reservist, he rose to Sous-lieutenant.
In the fall of 1893 Leclercq entered the Benedictine Solesmes Abbey, and made his vows on 15 January 1895. With Prior Fernand Cabrol and other monks he was sent, in 1896, to Farnborough, in the south of England, where former empress Eugénie de Montijo had founded Saint Michael's Abbey. Here he was ordained as priest on 24 August 1898. Prompted by Cabrol and with his assistance (though the seeds were probably planted in Solesmes already) he began to write the substantial historical books he became known for, including the Dictionnaire d'archéologie chrétienne et de liturgie, which he edited by himself after the death of Cabrol. To complete these volumes he spent more and more time in London, in the reading room of the British Museum. This in turn led to an appointment in the Italian hospital of Queen's Square; later he moved to the house of the Congregation of Our Lady of Sion in Bayswater, and was canonically released from the Benedictines to join the clergy of the Roman Catholic Diocese of Westminster.
His works are judged variously by scholars, though they are considered useful still because of their wealth of primary material. His final manuscripts weren't published until after the end of World War II.
See also Klauser, pp. 137-144.