An integrated geophysical survey was conducted in September 2007 at the Cathedral of Tarragona (Catalonia, NE Spain), to search for archaeological remains of the Roman temple dedicated to the Emperor Augustus. Many hypotheses about its location have been put forward, the most recent ones suggesting it could be inside the present cathedral. Tarragona’s Cathedral, one of the most famous churches in Spain (12th century), was built during the evolution from the Romanesque to Gothic styles. As its area is rather wide, direct digging to detect hidden structures would be expensive and also interfere with religious services. Consequently, the use of detailed non-invasive analyses was preferred. A project including Electrical resistivity tomography (ERT) and Ground probing radar (GPR) was planned for a year and conducted during a week of intensive field survey. Both ERT and GPR provided detailed information about subsoil structures. Different ERT techniques and arrays were used, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data were recorded extensively, making available many thousands of apparent resistivity points to obtain a complete 3D image after full inversion. The geophysical results were clear enough to persuade the archaeologists to excavate the area. The excavation confirmed the geophysical interpretation. In conclusion, the significant buried structures revealed by geophysical methods under the cathedral were confirmed by recent archaeological digging as the basement of the impressive Roman Temple that headed the Provincial Forum of Tarraco, seat of the Concilium of Hispania Citerior Province.