Travelers’ works provide us with very wealthy data including maps, engravings, drawings, inscriptions, accounts, stories etc. These data answer many questions when studying historical buildings; esp. for those which destroyed completely or partially. In this context this paper aims to identify the Hadji ‘Ali Mosque (the so-called KüçükCami). It is located in the south-eastern part of the Roman Agora of the old City of Athens. This monument is almost completely absent in any historical-archaeological map of Athens and in the published studies of the Ottoman monuments of Athens as well. Moreover the surviving monuments of Ottoman architecture are in general little known and hardly studied. So this article also aims to clarify, as far as possible, the history and architecture of an Athenian mosque, the Hadji ‘Ali, through the surviving traces of the building and with the help of the data provided by many accounts of travelers to Athens.