According to ancient literary tradition, Utica is considered to be one of the first three Phoenician foundations in the Western Mediterranean, supposedly founded in 1101 BC by Levantines from Tyre. In the Phoenician and Roman periods, it was an important merchant coastal town, on a promontory facing the sea. Over the centuries Utica lost its access to the sea, and its ports silted up as a consequence of the activity of the wadi Medjerda, which flowed to the south of the city. Despite over a century of investigation by archaeologists and associated researchers, the location of the city's harbour structures from the Phoenician and Roman periods remains unknown, buried under sediments resulting from the progradation of the Medjerda. Based on the study of sedimentary cores, the research presented here highlights the existence of a long maritime façade to the north of the Utica promontory in Phoenician and Roman times. A deep-water marine environment is attested in the former bay from the 6th mill. BC and the depth of the water column along the northern façade was still 2 m around the 4th-3rd c. BC. Another core to the east of the Kalaat El Andalous promontory showed the possibility that this sector was a sheltered harbour during the Phoenician and Roman periods. This paper illustrates the contribution of geoarchaeology to address this archaeological problem and to understand the relations of this important port city with the sea.