The sculptural program at St. Sernin is extensive and complex, with a large series of fragments, reliefs, and historiated capitals that have been reset and reused in a variety of contexts. The sculpted portal on the south side of the nave is known as the Porte Miègeville, and depicts the Ascension of Christ. The lintel depicts the twelve Apostles flanked by two angels in active poses of adoration as they gaze up towards the ascending figure of Christ. A foliate band divides the lintel from the tympanum, where Christ is depicted flanked by angels. Four angels stand in reverence with hands raised. Two others grasp Christ by the torso and work to bodily lift him as he ascends, a rare composition in Romanesque images of the Ascension. The composition looks instead to early Christian prototypes, for instance the fifth century portal of St. Sabine in Rome. Christ's halo is inscribed with the words "REX" and "DEUS PATER." These refer both to Christ's status as King, and also to his existence as an image of God the father on earth.
The corbels on the left and right of the tympanum are historiated. The left shows an image of King David, an Old Testament ancestor of Christ, and the right shows two men riding lions. Three of the four capitals of the embrasures are also historiated, showing the Expulsion from Paradise, the Annunciation and Visitation, and the Massacre of the Innocents. They are sculpted on all four faces, and thus were probably not originally intended for this location. In the spandrels on each side of the tympanum are large standing reliefs, St. Peter on the right and St. James the Greater on the left. St. Sernin is on the pilgrimage route to Compostella, which explains why James would be a pendant to Peter, instead of the more common figure of Paul. These two reliefs were also possibly originally intended to be displayed elsewhere, and were put in their current location under the 19th century restorations of Viollet-le-Duc.
The Porte Miègeville was probably commissioned at the beginning of the twelfth century by Raymond Gayrard, the celebrated canon and maître d'oeuvres of St. Sernin. Its style, with active poses and sweeping, energetic drapery, corresponds to the workshop of Bernard Gilduin, responsible for other sculptures in the basilica as well. Gilduin was particularly known for his appreciation of antique figural styles, known most notably through Carolingian ivory carving.
The Porte des Comtes on the entrance to the south transept displays eight historiated capitals depicting the story of Lazarus and Dives, Luxuria, Avaritia, and men held captive by demons and dragons. The themes of these capitals continue on the interior of the transept, performing a sort of processional continuity from exterior to interior. The spandrel between the two arched doorways originally held a relief of St. Saturninus, for whom the church is named, flanked by two lions, still in situ. These three images together would have served to protect the doorway. Two other saintly reliefs would have flanked the outer doorways, however they are damaged beyond recognition. This type of iconography composed of inset relief plaques was common to the region through the eleventh century.
The interior of the basilica also contains important sculpted elements. Besides a large quantity of sculpted and historiated capitals in the nave, chevet, and gallery, the ambulatory retains seven beautiful relief panels set into its walls. Their original placement isn't known, and it's possible they even came from a different church entirely, however they were set in their current location in the nineteenth century. Four of the panels depict angels, two depict apostles, and one shows Christ in Majesty. They show a powerful decorative stylization of pose and drapery typical of the Romanesque. There are strong parallels to antique, Byzantine, and Carolingian ivory carving. Two different workshops can be discerned among the panels, including the workshop of Bernard Gilduin.