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 Constantinople, he was educated at the theological school in Khalki, Greece, and at the universities of Leipzig, Munich, and Berlin. He became a professor at Chalki in 1861, and then director in 1863. 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 1877 he transferred as Metropolitan to Nicomedia.
In 1877, he participated in a commission dealing with plundered monasteries in Moldavia and Wallachia.
Metropolitan Bryennios died in the year 1917 in his native Constantinople.
While in Constantinople, he discovered a manuscript in the Jerusalem Monastery of the Most Holy Sepulchre, which contained a synopsis of the Old and New Testaments arranged by 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 spurious letter of Mary of Cassoboli, and twelve pseudo-Ignatian Epistles. The letters were published in 1875, and the Didache in 1883, and the letters of Clement and the Didache had notes written by Metropolitan Bryennios himself. The discovery of the Didache was significant because early 3rd, 4th and later century writers spoke of it, but it was presumed lost.