Saint Theonistus was an archbishop and martyr whose historical details are often contradictory due to conflicting medieval accounts. Living around the late 4th or 5th century, he is variously identified as a bishop of Namsia or Philippi. He is traditionally associated with his companions, Tabra and Tabratha, and is said to have been martyred in Gaul (Gallia) by Arians. His life and the timing of his death are subject to different chronological interpretations in historical documents.