Arethusa (or Aretusa) is a Roman Catholic titular see in the former Roman province of Syria, near Apameia. The modern, Arabic name of the site is Er-Rastan.
It has been used as a titular episcopal see in both the Roman Rite and the Syrian Rite, but is presently vacant in both.
Its episcopal list (325-680) is given in Gams (p. 436). It was also a Latin see for a brief period during the Crusades (1099–1100).
In the time of Roman Emperor Flavius Julius Constantius (337-361), its Bishop — later Saint — Marcus of Arethusa, destroyed a heathen temple which under the apostate Emperor Julian he was ordered to rebuild. To avoid this he fled from the city, but eventually returned to save the Christian people from paying the penalty in his stead, and underwent very cruel treatment at the hands of the pagan mob (Sozomen, Historia Ecclesiastica, x, 10) in 362. He is said to have been the author of the Creed of Sirmium (351) and is counted by Tillemont as an Arian in belief and in factious spirit. He was struck from the Roman Martyrology for years, but research by the Bollandists vindicated him and restored his name to the roles; his liturgical Memorial is on March 29.