Ambrosiaster or Pseudo-Ambrose is the name given to the unknown author of a commentary on the epistles of Saint Paul, written some time between 366 and 384 AD. The name "Ambrosiaster" (literally in Latin: "would-be Ambrose"). Various conjectures have been made as to Ambrosiaster's true identity, and several other works have been attributed to the same author, with varying degrees of certainty.
Pseudo-Ambrose was the name given by Erasmus to refer to the author of a volume containing the first complete Latin commentary on the Pauline epistles. Alexander Souter has established that the same author wrote the Quaestiones Veteris et Novi Testament, which had long been attributed to Saint Augustine. Other works ascribed to the same author, less definitely, are the Lex Dei sive Mosaicarum et Romanorum legum collatio, De bello judaico, and the fragmentary Contra Arianos sometimes ascribed to the pseudo-Hilary and the sermo 246 of pseudo-Augustine.