1. Praefectus Praetorio of the East, under Constantius II., possessed great influence with this emperor. He had previously enjoyed the title of Comes, and as such was sent by Constantius on an embassy to his brother Constans at Petobio in Pannonia, in A. D. 348 (Athanasius, Apol. ad Constant. init.). As praefect of the East he did all in his power to excite the bad passions of Gallus, and to inflame Constantius against him. Thalassius died in A. D. 353, and was succeeded by Domitian (Amm. Marc. 14.1, 7; Zosim. 2.48). Godefroy maintains that Thalassius could not have died earlier than A. D. 357 because he is said to have been at the conference at Sirmium, which is usually placed in this year, and because the name of Thalassius, praefectus praetorio, occurs in a law dated A. D. 357. But Tillemont has shown that the conference at Sirmium ought probably to be referred to the year 351; and as Ammianus expressly places the death of Thalassius in A. D. 353, the Thalassius mentioned in the law may have been praefectus praetorio of Illyricum. The matter is discussed by Tillemont, Histoire des Empereurs, vol. iv. note xxix. sur Constance.

This Thalassius appears to have written some work on the history of his own times, as Suidas (s. v. Θεόφιλος) quotes his testimony respecting his contemporary Theophilus.