"Ambrosiaster" is the name given by Erasmus to an anonymous fourth-century commentator on the Epistles of Saint Paul whose work often circulated under the name of AMBROSE, though also under the names of AUGUSTINE and "Hilarius" (see CPPM IIA.1745). Souter's attribution (1905) of a set of Quaestiones on the Old and New Testaments to the same author has been universally accepted (for other minor works doubtfully attributed to Ambrosiaster, see Mara, Pat 4.184-6).

Despite much speculation, Ambrosiaster remains unidentified (for a list of some of the proposed candidates, see Mara, Pat 4.180). His principal works can be dated by internal evidence to the pontificate of DAMASUS (366-384). He probably lived in Rome, but some evidence points to periods of residence in northern Italy and in Spain. He uses frequent illustrations from the law and was interested in legal principles and institutions (see Souter 1927 pp 68-70). He was also familiar with Jewish apocrypha and traditions ( Souter 1927 pp 72-75), and may have been a convert from Judaism or paganism.