The St Seurins’s basilica as we know it nowadays is the result of various construction stages and reshaping since the late roman period. Our principal concern is the study of the most ancient parts still visible of this major religious building of Bordeaux. Our aim is to understand how and when its successive transformations occurred in the Early Middle Ages. We undertook an interdisciplinary research crossing building archaeology, archaeomaterial studies and physical chronology in a narrow space of the basilica, the crypt, which was originally a funerary area since the late Antiquity. Far from routine operations, the investigations in Saint Seurin contributed to the development and the validation of a novel dating technique of archaeological lime mortars by Single Grain Optically Stimulated Luminescence (SG-OSL). Archaeological observations and mortar analyses (macroscopic facies, mineral and elemental composition, chronology) allowed us to propose a consistent sketch of the progressive laying out of the crypt between the end of the 4th and the 10th century. Thanks to a detailed observation of the stratigraphy associated with the study of archaeomaterials, we document the building transformation that was the result of a change of a funerary memory into a religious practice controlled by the Church. The change of the function of the site implied an evolution of the construction that integrated a former mausoleum as a reused structure. This early medieval building is at the origin of the construction of the St Seurin church and the subsequent development of new settlements which were included lately during the post-medieval times in the Bordeaux urban area.