Ammonius may refer to:
- Ammonius Lithotomos (3rd century BC), Greek lithotomist
- Ammonius of Athens (1st century AD), philosopher and teacher of Plutarch
- Ammonius Saccas (3rd century AD), Neoplatonist philosopher and teacher of Plotinus
- Ammonius of Alexandria (Christian) (3rd century AD), Christian writer confused with Ammonius Saccas
- Ammonius Hermiae (5th century AD), Alexandrian philosopher
- Ammonius Grammaticus, Supposed author of a grammatical treatise
- Ammonius (genus), a genus of the spider family Barychelidae
- Ammonius (crater), a lunar crater
- Ammonius (monk)