| Kato Poseidonia | |
| Name Local: | Κάτω Ποσειδωνία |
| Coordinates: | 37.6667°N 27°W |
| Periph: | Attica |
| Periphunit: | East Attica |
| Municipality: | Lavreotiki |
| Municunit: | Lavreotiki |
| Population As Of: | 2021 |
| Population: | 337 |
Kato Poseidonia (or Kato Posidonia) is a settlement in Southeastern Attica under the administration of the Municipality of Lavreotiki. The settlement is 36.7 miles from Athens, 2.5 miles from Lavrio, and 3 miles from Sounio.