The Thermaic Gulf () is a gulf of the Aegean Sea located immediately south of Thessaloniki, east of Pieria and Imathia, and west of Chalkidiki (prefectures of Greece). It was named after the ancient town of Therma, which was situated on the coast along the gulf (Therma was later renamed Thessalonica). The length is about 100 km and the width is 5 km near Thessaloniki, about 15 km to the north and as far as about 50 km to the south.

To the Romans, the gulf was known as Thermaicus or Thermaeus sinus ("Gulf of Therma") or Macedonicus sinus ("The Gulf of Macedon" or "The Macedonian Gulf"). One of its modern names is the "Gulf of Salonica", after Thessalonica.

Places that lie within the gulf are Sani, Kassandreia, ancient Potidaea, Nea Moudania, Agia Triada, Neoi Epivates, Peraia, Thessaloniki, nea Kalochori, Methoni, Pydna, Paralia and Olympiaki Akti, Agia. The rivers emptying into the gulf are Pineios River, Aliákmon and Axios rivers. Several streams including one near Pydna empties into the gulf. Beaches including Sani Beach are around the gulf while the wetland featuring swamp grass is in the northern part stretching from near Methone east into near Kalochori and some within the Peneus.

The Pierian plain and the plain to the south surround the gulf while mountains and hills dominate the east and the northwest. Ports around the gulf including twelve small ports and a major port in Thessaloniki. Cape Kassandra lies to the east. GR-1/E75 (Athens - Lamia - Thessaloniki) encircles the western portion of the gulf and GR-18 (Thessaloniki - Kassandra) almost encircles in the east.

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