Petros 9 was an anagnostes and notarios in the patriarchal secretariate at Constantinople; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) where he was one of the official readers and is attested at the first, fourth and fifth sessions, on 24 September, 1 October and 4 October 787; he is styled Πέτρος ὁ εὐλαβέστατος ἀναγνώστης καὶ νοτάριος τοῦ εὐαγοῦς πατριαρχείου or similar: Mansi XII 1027 (τοῦ εὐαγοῦς πατριαρχικοῦ σεκρέτου), XIII 69, 85, 176. At the first session he read out passages from the writings of Cyril of Alexandria: Mansi XII 1027. At the fourth session he read out passages from Athanasios and from the Life of Maria the Egyptian: Mansi XIII 69, 85. At the fifth he read from a work by Amphilochios of Ikonion: Mansi XIII 176.

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