Sisinnios 17 was bishop of Chalkis (in Thrace); in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 995-XIII 486. In the list of those present at the first and seventh sessions he is styled Σισιννίου ἐπισκόπου Χαλκίδος: Mansi XII 995 (Χαλκηδόνης τῆς Θρᾴκης), XIII 368. The Latin version at XII 996, reads: "Chalcedonis Thraciae", with Chalcidis as a variant reading. Elsewhere he is styled Σισίννιος ὁ ὁσιώτατος ἐπίσκοπος Χαλκίδος or similar: Mansi XII 1099, XIII 144, XIII 388 (all versions, Greek and Latin, have Χαλκίδος or "Chalcidis"). In the lists he is named in a group of bishops from Europa. At the second session he supported the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1099. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 144. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 388.

He is perhaps identical with the bishop of Chalkis, Sisinnios, one of whose seals survives: DOSeals I 48.2 = Laurent, Corpus V 1, no. 329 (obv.: bust of the Virgin holding medallion of Christ, and a monogram of [Θεοτόκε] βοήθη; rev.: Σισιννίῳ ἐπι[σ]κόπῳ [Χαλ]κίδος). Laurent dates it to the late ninth century, Nesbitt and Oikonomides favour the first half and note features typical of the period between the First and Second Iconoclastic periods (787-815) (e.g. the presence of the Virgin between two cruciform monograms and the form of the beta), adding however that other features point to a later period; they prefer the earlier date, c. 800, on balance; see Nesbitt and Oikonomides, DO Seals I, pp. 131-132, note.

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