Sometimes referred to as 'the Crypt', St. Mary Undercroft is a chapel in which London landmark?

The Chapel of St. Mary Undercroft is a Church of England chapel in the Palace of Westminster.

It had been a crypt below St Stephen's Chapel and had fallen into disuse, being pressed into service at various times as a wine cellar, dining room for Speakers (who had holes bored into the wall to accommodate two kitchen chimneys) and (now unconfirmed by records) stables for Oliver Cromwell's horses.

After a fire had destroyed St Stephen's Chapel in 1834, the undercroft returned to its former use as a place of worship. Although much stonework was damaged in the fire, it was decorated in the 1860s by Edward Middleton Barry with gilded, painted and stenciled designs in rich colours to cover the walls, floor and vaulting. The backdrop of the altar depicts royal British saints.

It is still used for worship purposes today. In particular, children of peers, who possess the title of "The Honourable", have the privilege of being able to use it as a wedding venue. In addition, members of parliament and peers have the right to use the chapel as a place of christening in the baptistery and font (whose basin was made from a single slab of alabaster) designed by Barry.