For the Franco-Irish saint, see Sidonius of Saint-Saëns.

Gaius Sollius (Modestus) Apollinaris Sidonius or Saint Sidonius Apollinaris (November 5 of an unknown year, perhaps 430 – August 489) was a poet, diplomat, and bishop. Sidonius is "the single most important surviving author from fifth-century Gaul" according to Eric Goldberg. He was one of four Gallo-Roman aristocrats of the fifth- to sixth-century whose letters survive in quantity; the others are Ruricius bishop of Limoges (died 507), Alcimus Ecdicius Avitus, bishop of Vienne (died 518) and Magnus Felix Ennodius of Arles, bishop of Ticinum (died 534). All of them were linked in the tightly bound aristocratic Gallo-Roman network that provided the bishops of Catholic Gaul. His feast day is August 21.