This study provides the first detailed insight into the composition and properties of structural mortars used in a 4th‐century AD bath complex in Aquileia, the emblematic center of Roman culture in Northern Italy. Eighteen mortars, taken from different structures of the site, and three stone samples from the vaulting opus caementicium have been analyzed adopting a multianalytical approach integrating optical microscopy, X‐ray powder diffraction, X‐ray fluorescence, and scanning electron microscopy coupled with energy‐dispersive spectroscopy. The properties of the compounds are outstanding, as revealed by the formation of hydraulic phases (i.e., Al‐tobermorite and AFm) in most of the samples: the waterproofing capabilities of cocciopesto mortars are remarkable, as revealed by the formation of anthropogenic Al‐tobermorite (5.5 wt%) in pool coating samples; the lightweight of the vaults was guaranteed by the use of porous caementa and pozzolanic volcanic aggregates imported from the Gulf of Naples, as demonstrated by petro‐mineralogical features and chemical analysis of major and trace elements. This is the first proven case of trade in these building materials to the north of the Italian peninsula. These outcomes shed new light on the robust technical expertise of local artisans in Aquileia and indicate that the Cisalpina province was by no means a peripheral reality in the Roman Empire, as far as mortar‐based materials are concerned.