The Basilica of Saint-Quentin (French: Basilique Saint-Quentin), formerly the Collegiate Church of Saint-Quentin (French: Collégiale Saint-Quentin) is a Catholic church in the town of Saint-Quentin, Aisne, France. There have been religious buildings on the site since the 4th century AD, which were repeatedly destroyed and rebuilt during the Early Middle Ages. The present basilica was constructed in stages between the 12th and 15th centuries. It was severely damaged in World War I (1914–18), and was only reopened in 1956 after extensive reconstruction.

The town of Saint-Quentin has been identified with the Roman city of Augusta Veromandurorum, a commercial center at an important crossroads. It takes its present name from the Christian missionary Saint Caius Quintinus, who was beheaded there in 287 AD. Legend says the body was found many years later in the nearby marches of the River Somme by a Roman widow named Eusebia. She reburied the remains at the top of the hill at the center of the present town and built a small shrine to the martyr. Excavations round the crypt of the present church have indeed found traces of a building from this date.

Some sources say the town became the seat of a bishopric around 365, but after barbarians destroyed it in 531 the bishop moved to Noyon. The chapel is listed as a pilgrimage destination by Gregory of Tours (c. 538–594). Saint Eligius (c. 588–660), Bishop of Noyon and counselor to Dagobert I, Merovingian king of France, rediscovered the tomb in the 7th century "under the pavement of the basilica". There are records of Saint Eligius having enlarged the building. Remains of a floor from this period have been found near the crypt. A large fragment of mosaic has been preserved.

The first community of monks was established in or around Saint-Quentin by the mid-7th century, probably by Irish monks with the backing of the bishops of Noyon. The bishops claimed ecclesiastic jurisdiction over most of Vermandois, including Saint-Quentin. The church was rebuilt with the assistance of Charlemagne (c. 747–814), and consecrated by Pope Stephen IV (r. 816–817). There may have been two churches in the early Middle Ages, one dedicated to Saint Quentin and the other to the Virgin Mary. Archaeologists have found the remains of walls from the Carolingian period, when the location was a flourishing monastery and pilgrimage site. Limited archaeological investigations indicate that the crypt in the section of the nave between the two transepts may have replaced a Carolingian crypt with a circular corridor.

The church was burned down by the Normans in 816, rebuilt in 824 and burned down again in 883. The site was fortified after 883. In 900 the bodies of Saint-Quentin and two other saints were placed in stone sarcophagi in a newly constructed crypt. In the 10th century the Herbertian counts of Vermandois, principally Adalbert I (Albert the Pious c. 915–987), replaced the monks with a congregation of secular canons. The counts claimed the title of abbot, and were overlords of the church. A dean administered the collegiate church. The dean was appointed by the count and reported only to the count, not to the bishop of Noyon and then to the archbishop of Reims. There are records of rebuilding in the mid-10th century. The church was damaged during local warfare in 1102–03. At one time Count Raoul of Vermandois (c. 1100–52) was thought to have rebuilt the church, but this now seems unlikely.

The canons of the collegiate chapter lived in separate houses within the church precincts, and most were not ordained priests. Often they were property owners and businessmen from noble families and main role was to administer the chapter's property. By 1200 the chapter was large and prosperous, with 72 canons. The semi-independent counts of Vermandois were originally closely associated with the County of Flanders, but gradually came under French control, a process that was completed when Eleanor of Vermandois died in 1214. That year Cardinal Robert of Courçon instituted a reform whereby ordained parish priests in nine parish churches would administer the sacraments, independent of the chapter, although they would pay the chapter a portion of their fees for baptisms, marriages and funerals. The construction of the huge new collegiate church may have been motivated in part by a desire of the collegiate chapter to reassert its authority after loss of jurisdiction to the town authorities and the parish priests.

A tower, now the bell tower, was started around 1170. It was completed between 1195 and 1200. Around 1190 the canons decided to build a more imposing church to welcome the growing numbers of pilgrims. The apse was completed in 1205. The apse was some distance to the east of the tower. Construction then continued westward from the apse towards the 12th-century tower, which eventually became the entrance to the collegiate church.

There were frequent disputes between the chapter and the Bishops of Noyon, whose authority they refused to recognize. In the early 1220s Bishop Gérard de Bazoches placed the chapter under interdict, but in 1228 Pope Gregory IX rescinded the interdict. That year Nicolas de Roye, who was related to several of the canons, became the new bishop after Gérard's death. Later in 1228 the relics of Saint Quentin, Saint Victoric of Amiens and Saint Cassian of Auxerre were moved from the old crypt to a temporary location in the nave. The remains of the saints were exposed for veneration. Their heads, hands and arms were detached from their bodies, and each body part was placed in its own reliquary. After completion of the sanctuary the reliquaries were placed behind the high altar. The crypt, its tombs now empty, was preserved as a sacred place.