The synagogue at Ostia is one of the most important ancient synagogues. However, no final report from the excavations yet exists, which has left the field open for some debate as to the origin and nature of the building. Apart from discussing matters of terminology and definitions, the present study provides a detailed analysis of the building's architectural development, relating it to other structures at Ostia. It is maintained that the original edifice was public and monumental, containing a triclinium for common meals, and dated to the late Julio-Claudian period. The results of the analysis have implications for how we understand Jewish identity as expressed in architectural form, as well as the relation between Jews and non-Jews in the Diaspora.