This research provides the first detailed insight into the composition of hydraulic bedding and coating mortars used in the Roman city of Hippo Regius with the aims of bringing to light the use of natural pozzolanic aggregates for mortars preparation following the legacy of Vitruvius and determining their possible sources. Nine samples of hydraulic mortars taken from different structures have been thoroughly analysed adopting a multi-analytical research strategy combining thin-section petrography, optical cathodoluminescence (CL) microscopy, and scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). The results indicate the use of pumice clasts with trachytic composition (SiO2 : 58.9-64.4 wt.% and total alkali oxide content: 8.81-14.20 wt.%) as a pozzolanic additive to enhance the strength and hydraulicity of mortars. The comparison of petro-mineralogical features and major element concentrations with literature data reveals the Campi Flegrei caldera (Phlegraean Fields) in the Gulf of Naples, and probably the Baia-Fondi di Baia, as possible area of provenance. This is the first proven case of trade in Phlegraean tephra as a building material to a city of Africa Proconsularis, and a new piece of evidence of the spread of Roman technologies to the southern Mediterranean region of the Roman Empire as well as the transfer of their technical expertise to local artisans of the provincial city of Hippo Regius.