Saturnin of Toulouse (Latin: Saturninus, Occitan: Sarnin, French: Saturnin, Sernin, Catalan: Serni, Sadurní, Galician: Sadurninho and Portuguese: Saturnino, Sadurninho, Basque: Satordi, Saturdi, Zernin, and Spanish: Saturnino, Serenín, Cernín) was one of the "Apostles to the Gauls" sent out (probably under the direction of Pope Fabian, 236–250) during the consulate of Decius and Gratus (250–251) to Christianise Gaul after the persecutions under Emperor Decius had all but dissolved the small Christian communities.

Fabian sent out seven bishops from Rome to Gaul to preach the Gospel: Gatien to Tours, Trophimus to Arles, Paul to Narbonne, Saturnin to Toulouse, Denis to Paris, Austromoine to Clermont, and Martial to Limoges.

Two Christian women (puellae remembered as "les Puelles") piously gathered up the remains and buried them in a "deep ditch", that they might not be profaned by the pagans.

[1] It is not beyond possibility, in this part of Gaul, where even today the greatest bull among many in Toulouse is honored with the name "Le Grand Taur", that the deep ditch was in fact a mithraeum.

410), gave him more honorable burial, once Christian rites were no longer illicit, by erecting a simple wooden oratory over the "Roman crypt" (as modern guides describe it) where he had been interred.

At the end of the century, the press of pilgrims to the cramped site encouraged Bishop Silvius (360–400) to build a larger church, finished by his successor Exuperius (Exupère) (400 – ?)