Philotheos Bryennios (Greek: Φιλόθεος Βρυέννιος; 7 April 1833 – November 18, 1917) was a Greek Orthodox metropolitan of Nicomedia, and the discoverer in 1873 of an important manuscript with copies of early Church documents.

Born in the Tavtalos (Kurtuluş) district of Constantinople, with the secular name of Theodore, he was educated at the theological school in Halki, and at the universities of Leipzig, Munich, and Berlin.

In 1867 he went to head the Patriarchal School in Constantinople, leaving in 1875 to attend the Old Catholic conference in Bonn, during which he was appointed metropolitan of Serres in Macedonia.

In 1873, he discovered a manuscript in the library of the monastery of the Holy Sepulcher (Jerusalem Patriarchate metochion) in Constantinople (present day Istanbul, Turkey), which contained a Synopsis of the books of the Old and New Testaments attributed to St. John Chrysostom, the Epistle of Barnabas, the First Epistle of Clement to the Corinthians, the Second Epistle of Clement to the Corinthians, the Teaching of the Twelve Apostles (Didache), the twelve Ignatian Epistles, and a short treatise on the genealogy of Christ.

The discovery of the Didache was significant because writers of the early 3rd, 4th and later centuries had spoken of it, but it was presumed lost.