The Basilica of St. Nazaire and St. Celse or St. Nazarius and St. Celsus (Basilique Saint-Nazaire-et-Saint-Celse de Carcassonne), is a basilica minor in Carcassonne, southern France, listed as a national monument of the country.
It was formerly the cathedral of Carcassonne until 1801, when it was replaced by the present Carcassonne Cathedral (Cathédrale Saint-Michel de Carcassonne).
The present church is in origin a Romanesque of the 11th century, consecrated by Pope Urban II in 1096. It was built on the site of a Carolingian cathedral, of which no traces now remain. The crypt too, despite its ancient appearance, dates from the new construction. The church was enlarged between 1269 and 1330 in the Gothic style then predominant in France, largely at the expense of the Bishop of Carcassonne, Pierre de Rochefort.
The exterior was largely renewed by Viollet-le-Duc, while the interior has largely remained the Gothic original.