Thalassios 1 was an hegoumenos and priest; he was hegoumenos of the monastery of the Armenians at Rome, at the monastery of Renatus and apparently also a priest in the church of the Mother of God and St Andrew; he was one of the monks who signed the libellus attacking monothelete views, which was read out to the second session of the Lateran Council on 8 October 649; the signatories were admitted to the session while the document was read out: Conc. Lat., p. 50, lines 4-5 (Θαλάσσιος πρεσβύτερος καὶ ἡγούμενος τῆς εὐαγοῦς μονῆς τῶν Ἀρμενίων τῆς ἐνθάδε παροικούσης εἰς τὴν ἐπιλεγομένην μονὴν Ρενάτου), p. 51, lines 3-5 ("Thalassius abba presbiter venerabilis monasterii Armenissarum in hac Romana civitate constituti qui appellatur Renati"), p. 57, no. 2 ("Thalassius misericordia dei abba presbiter sanctae dei genetricis et beati Andreae"), cf. p. 48, line 28, p. 49, lines 27-28.

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