This article focuses on the built spaces, often described as mosques, of two Muslim communities in Constantinople between the tenth and thirteenth centuries. These Islamic spaces in the Byzantine capital originated pragmatic solutions to the functional requirements of accommodating Muslim prisoners and merchants. During this period one of these built spaces acquired political status as “the mosque of Constantinople” in diplomatic negotiations, serving as a counterpart to Christian monuments in Islamic territories. By the end of the twelfth century the Muslim spaces of Constantinople had acquired social, economic and religious significance for an international Muslim community, becoming in effect Islamic monuments. The “mosques” of Constantinople thus illuminate a role for architecture within Byzantine-Islamic diplomatic exchange, and a process of medieval monument formation, based not on intrinsic artistic interest, but on meanings acquired through social processes.