Excavations at the Latin city of Gabii in 2012–15 conducted by the Gabii Project have uncovered a monumental building complex, hitherto known only very partially from previous excavations in the 1990s. Organized on a series of three artificial terraces that regularized the slope of the volcanic terrain, it measures some 60 m by 35 m, occupying an entire city-block. It is prominently situated at one of the most central locations within the city, on the main urban thoroughfare at the important intersection of the roads from Tibur, Praeneste and Rome. Stratigraphic evidence and construction techniques date the original phase of the building to the mid-third century BC. This report focuses on a contextualization and description of this first, mid-Republican phase and offers a preliminary interpretation of this complex as a public building, with spaces designed for a variety of functions: bathing, public feasting, and ritual activity. If this is correct, it now represents one of the very few examples of public buildings other than temples and fortifications known from the mid-Republican period, and sheds important light on the development of Roman architecture and of the Latin cities in a crucial and obscure period.