Theodosios 11 was bishop of Nyssa (Nysa) (in Asia); 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 lists of those present at the first and seventh sessions he is styled Θεοδοσίου ἐπισκόπου Νύσσης: Mansi XII 995, XIII 368. Elsewhere he is styled Θεοδόσιος ὁ ὁσιώτατος ἐπίσκοπος Νύσσης or similar: Mansi XII 1098, XIII 141, XIII 385. He usually appears among the bishops from Asia as distinct from those from Cappadocia, and his see is therefore to be identified with Nysa in Asia, not with Nyssa in Cappadocia (see Ioannes 88). At the second session he spoke in support of the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1098. At the fourth session he subscribed to the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 141. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 385.

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