Larissa (Greek: Περιφερειακή ενότητα Λάρισας) is one of the regional units of Greece. It is part of the region of Thessaly. Its capital is the city of Larissa.
Larissa is the second largest regional unit in Greece, exceeded only by Aetolia-Acarnania. It covers about one-third of Thessaly. It borders the regional units of Kozani to the northwest, Pieria to the northeast, the Aegean Sea to the east, Magnesia to the southeast, Phthiotis to the south, Karditsa to the southwest and Trikala to the west.
The tallest mountain in Greece, Mount Olympus (2,917 m) is situated in the northeastern part of the regional unit. Mount Ossa is situated in the east, at the Aegean coast. The lower stretch of the river Pineios flows through the Vale of Tempe, between Olympus and Ossa.
The northern part is covered with forests, but most of the regional unit is fertile land, the Thessalian Plain.
Larissa has a mainly Mediterranean climate with hot summers and mild winters. Winter is harsher in the mountains areas, notably the north and Mount Olympus. Larissa is often the warmest area in Greece in summer and the coldest in winter. The highest temperature ever recorded was 45.4 °C(2000) and the coldest was -21.6 °C(1968). Τhe city experiences very often temperatures over 40 °C.
Its main economy since the 1970s are manufacturing, businesses, communications and services. Agriculture is the second leading industry.