Martyrium of Saint Denis, Montmartre, crypt in the 18th arrondissement, Paris, France

The Martyrium of Saint Denis is a crypt at 11 rue Yvonne-Le-Tac in Montmartre, reached by a stairway found in 1611 during restoration of the chapel above. The present crypt was built in 1886–1887 under the convent of the Auxiliatrices du Purgatoire. Its white vaulted chamber holds a pale stone altar on two supports, a dark wooden cross, benches with light cushions, and an oval inscribed plaque on the wall.

The crypt is said to stand where Saint Denis, a 3rd-century Christian martyr, was beheaded. From the 5th century the slope of Montmartre became a pilgrimage site visited by Geneviève of Paris, and a chapel existed by the 9th century. In 1133 Queen Adèle of Savoy founded the Benedictine abbey of Montmartre, which owned the chapel; it was consecrated by Pope Eugene III, Bernard of Clairvaux and Peter the Venerable, and received Thomas Becket as a pilgrim in 1169. The present crypt was built in 1886–1887 under the convent of the Auxiliatrices du Purgatoire.

The crypt is connected to Saint Denis, the first bishop of Paris, whose story is tied to the faith of early Christians in France. Visitors today use the space for prayer and reflection, connecting with the religious past of the city.

The crypt is at 11 rue Yvonne-Le-Tac in the 18th arrondissement, on the slope of the Montmartre hill. The entrance is through the convent of the Auxiliatrices, so ring before going in. Inside, a low platform with the altar stands before rows of benches; the stone-tiled floor and vaulted ceiling keep the room cool.

On 15 August 1534, Ignatius of Loyola, Francis Xavier, Peter Faber and four companions pronounced vows of poverty and chastity in this crypt and promised to travel to Jerusalem; Faber, the only priest among them, received the vow while giving communion. The group became the Society of Jesus, approved in 1540.