An early rescue excavation in 1900 revealed part of a medieval building of the Dominican friary of Black Friars, London. Further archaeological work in the twentieth century revealed other parts of the building. Here, the authors consider the archaeological and architectural evidence, including a preserved in situ window and two relocated ex situ architectural features. Alfred Clapham suggested in a 1912 article in Archaeologia that the building was the Dominican provincial prior’s house; the present authors instead identify the ground-floor chamber as the late thirteenth-century chapter house. Construction of the friary (the second London Black Friars) began in 1278 or 1279 and the chapter house, funded by a will of 1281, was probably built later in the 1280s. The lower chamber was a well-lit, five-bay undercroft with a quadripartite vault rising from Reigate stone responds and Purbeck marble columns: this was probably the chapter house chamber. The hall-like chamber over was approximately 57ft by 28ft (17.3m × 8.5m) and may have been the library. The building may be the work of Robert of Beverley, the king’s master mason from 1260, perhaps in conjunction with Michael of Canterbury. French royal works of the thirteenth century (such as the lower chapel of the Sainte-Chapelle, Paris) may have served as inspiration.