The Gulf of Thessaloniki, Thermaikos Gulf, is the largest bay of the Aegean Sea. Wets Thessaloniki in the northern part, Pieria to the west and the east Halkidiki. The width ranges from 5 km in Thessaloniki until 50 in the southern part of.
In Thermaikos flow four rivers the Axios river, Aliakmonas, Loudias and French, and several smaller ones. In western coasts dominate the Pieria Mountains and Olympus.