In the East Cloister is the 'incomparable' octagonal Chapter House, dating from the 1250s. It is one of the largest in England. The monks met here every day for prayers and to read a chapter from the rule of St Benedict and discuss the day's work. The King's Great Council first assembled here in 1257. This was effectively the beginning of the English Parliament. The House of Commons regularly used the room in the 14th century, before they transferred to the Palace of Westminster. After having been a repository for government records from the 1540s it was restored in Victorian times by Sir Gilbert Scott.

The room is lavishly adorned with sculpture, and wall paintings of the Apocalypse, with the Last Judgement painted on the east wall. It contains one of the finest medieval tile pavements in England. The windows incorporate Victorian stained glass and new post-war designs. An inscription underneath them recalls the work of the original masons "In the handiwork of their craft is their prayer".

In the vestibule of the Chapter House is the oldest door in Britain, dated to the 1050s.