The cell occupied by St John of Matha is over the Arch of Dolabella. The chapel is very small. It is entered through an arched portal in a yellowish-white-in-grey brecciated marble. The interior walls are revetted in the same brecciated marble. The floor is in a yellow marble, with a deep red marble border. The sanctuary platform is in red marble inlaid with diaper square white tiles in a chequerboard pattern. Above the altar is a shallow round-headed niche in vermillion red, in which a portrait in oils of the saint is hung.