The Thermaic Gulf (Greek: Θερμαϊκός Κόλπος, Thermaïkós Kólpos), also called the Gulf of Thessaloniki and the Macedonian Gulf, is a gulf constituting the northwest corner of the Aegean Sea. The city of Thessaloniki is at its northeastern tip, and it is bounded by Pieria Imathia and Larissa on the west and the Chalkidiki peninsula on the east, with Cape Kassandra at the southeasternmost corner. It is named after the ancient town of Therma, modern Thessaloniki. It is about 100 km (62 mi) long.
By narrower definitions, the Thermaic Gulf is bounded on the west by the line from the mouth of the Axios or Vardar to Cape Megalo Embolo, making it about 15 km (9.3 mi) long; while the smaller Gulf of Salonica is bounded by a line running from the mouth of the Gallikos to Mikro Emvolo.