Sardis constituted a major urban centre during the early Byzantine period, an era marked by the gradual transformation of the city into a Christian metropolis. From the fourth to the sixth century AD, Sardis maintained an important commercial, industrial and administrative role that sustained high-quality monumental construction. Yet, the major architectural types that emerged during this crucial period in the centre of the early Christian city are largely unknown to us. The unexcavated remains of the monument known as ‘Building D’ offer the best opportunity to shed light on this enigmatic aspect of the history of the city. Indeed, these remains display a late fourth century reused inscription as well as construction details typical of the early Byzantine period. They also lie in what must have been a central area of the city during this time. At first sight, this building seems hopelessly dilapidated and largely inaccessible. Still, the current paper demonstrates that a closer examination of the fabric of Building D reveals invaluable clues for its original form and function. This new exploration of the building includes the graphic recording, careful analysis and interpretation of the remains, thus providing the evidence required for the first substantiated reconstruction of a major part of the monument. The exploration reveals the articulation and structure of the primary load-bearing elements, as well as the form of the enormous vaulted canopy that covered one of the most imposing and towering spaces of early Byzantine Sardis. The article uses this reconstruction as a basis for the identification of those architectural features that help to interpret the function of the building, its role in the development of late antique Sardis as well as its position in the evolution of early Byzantine architecture in western Asia Minor.