Although being one of the largest churches possibly of Protobyzantine era currently standing and in liturgical use, the Basilica Saint Sophia in Sofia is still – except in Bulgaria itself – too little taken into account in specialized historiography. The building indeed poses complex issues. We will first revisit its initial function. And, to the extent that, following a partial destruction, it underwent a radical reconstruction in the 20th century, we will re-examine its original design – especially its rather unique elevation. This obviously involves reconsidering the various proposals that have been made regarding its dating; and insofar as the hypothesis of a construction in the 6th century appears to be the most plausible, we will attempt to place this church within the broader architectural context of the period.