Gennadius or Gennadios may refer to:
- Gennadius I, Patriarch of Constantinople from 458-471 AD
- Gennadius II, Patriarch of Constantinople from 1454-1464 AD
- Gennadius of Massilia, 5th-century historian, best known for his work De Viris Illustribus
- Gennadius of Astorga, Saint and Bishop of Astorga (Spain) in the ninth century'
- Gennadius (6th century), Byzantine general and first exarch of Africa
- Gennadius (7th century), Byzantine general and exarch of Africa