Synkellos (Greek: σύγκελλος), latinized as syncellus, is an ecclesiastical office in the Eastern Rite churches.
It is attested from the 5th century onward for the closest advisor of a bishop or archbishop, who then lived in the same residence or cell.
This was certainly the expectation by the 9th–10th centuries, when the synkellos was an official appointed by the Emperor, and became a tool for imperial control of the patriarchal succession.
The synkellos was one of the "special dignities" (ἀξίαι εἰδικαί, axiai eidikai), between the rhaiktor, and before the chartoularios tou kanikleiou.
His exact duties are unclear, but he was considered a member of the Senate and was accorded a role in imperial ceremonies.