Magnesia (Greek: Μαγνησία, Magnisía, IPA: [maɣniˈsia], Ancient Greek: Magnēsía, deriving from the tribe name Magnetes) is one of the regional units of Greece. It is part of the region of Thessaly. Its capital is the city of Volos. About 70% of the population of Magnesia live in the Greater Volos area, which is the second-largest city in Thessaly and the third busiest commercial port in Greece.

According to the most recent census (2021), the population stands at 177,448. The regional unit hosts 2,000,000 tourists annually. Magnesia is represented in the Greek Parliament by six seats. Its main agricultural products are wheat, cotton, tomatoes, grapes, olives, apples and honey.

A prominent geographic feature of Magnesia is the Pagasetic Gulf, a bay of the Aegean Sea. The Pelion mountain range closes off the Gulf on the east and south side, leaving only a narrow channel near Trikeri. The highest peak of the wooded Pelion is Pourianos Stavros or Xeforti, (altitude 1,624 metres or 5,328 feet). On the south edge of Magnesia peninsula Tisaio mountain is found.

Mt. Maurovouni (1,054 metres or 3,458 feet) is the northeasternmost mountain of the regional unit and extends to the neighboring regional unit of Larissa. The southwest border of Magnesia is formed by the Othrys, with its highest peak Gerakovouni (1,726 metres or 5,663 feet). The interior of Magnesia has two plains. The plains southwest of the Pagasetic Gulf are called the Almyros plains, while the plains northwest of the Gulf are called the Volos-Velestino plain. The hydrological network of Magnesia is not particularly rich and is characterized by the absence of big rivers. The waters coming from Pelion shape the rivers Anavros, Platanorema, and Xirias.

In the North section of Magnesia is the shallow Lake Karla. Lake Karla was completely drained in 1962, but as of 2018 water had been restored to at least 50km2 of its former 180km2. On the bight of Sourpi, next to Amaliapolis a coastal wetland is located, with various species of migratory birds. This wetland together with the forest of Kouri – an infrequent lowland of oak forest close to Almyros – is included in the list of the protected regions of the European Network Natura 2000.

The average temperature is 17 degrees Celsius and the average rainfall about 540 millimetres per year. Heat waves and intense cold periods are rare. During the summer the temperature rises up to about 37 to 38 °C (99 to 100 °F) in August. The climate varies in different parts of the prefecture; close to the Pagasetic Gulf conditions are humid, in Nea Ionia it is quite dry and in Velestino and Almyros is the climate is continental. In winter there is significant snowfall in the mountains and often freezing temperatures.

The regional unit Magnesia is subdivided into 5 municipalities. These are (number as in the map in the infobox):

Magnesia was created as a prefecture. In addition to the territory of the present regional unit of Magnesia, the Magnesia Prefecture included the Northern Sporades group of islands (Skiathos, Skopelos and Alonnisos). As a part of the 2011 Kallikratis government reform, the prefecture was split into the Magnesia and Sporades regional units. At the same time, the municipalities were reorganised, according to the table below.