Turku Castle (Finnish: Turun linna, Swedish: Åbo slott) is a medieval structure in the city of Turku, Finland. Together with Turku Cathedral, the castle is one of the oldest buildings still in use in Finland. It is also the largest surviving medieval building in Finland. It was founded in the late 13th century and stands on the banks of the Aura River. The castle served as a bastion and administrative center in the historical region of Finland until the early 19th century. It played a role in power struggles within Sweden and the Kalmar Union and stood sieges, with additional battles fought outside its walls. The castle was at its peak in the mid-16th century, during the reign of Duke John of Finland and Catherine Jagellon. It lost its status as an administrative center in the 17th century, after Per Brahe's period as governor-general of Finland came to an end. Turku Castle is Finland's most visited museum. Visitations reach well over 100,000 people annually. Some of the rooms in the castle are used for municipal functions.

The castle was the center of the historical province of Finland Proper, and the administrative center of all of Finland.

The construction of the fort began at around the year 1280. Swedes stationed in the region constructed it as a military fortress and outpost. The fort's defenses were bolstered and living quarters constructed during the following two centuries. The castle served as a defensive structure and administrative center in Österland, which was the name of the region known today as Finland, until the end of the 15th century. The main part of the castle was extended considerably during the 16th century, after Gustav Vasa had ascended to the Swedish throne. His son, John III of Sweden, was the head of the administration and duke of Finland at that time. The castle was generally improved upon during this time, with an addition of a tower at the southeast corner of the castle. These were the last additions to the main structure of the castle, and all work on the castle since then has focused on repairs and reconstruction.

Between 1395 to 1398, Turku Castle may have been used as a base of operations by the Victual Brothers, a band of privateers turned pirates, invited there by the chief of the castle, Knut Bosson Grip [fi] due to some larger international political intrigue and allegiances concerning trade in the Baltic Sea. In Paulus Juusten's episcopal chronicle, it is said that at the turn of the 15th century, pirates were causing havoc in and around the Turku archipelago. They looted the Turku Cathedral, a historical building in the center of Turku, also on the Aura river. It is on the opposite side of the town away from the sea. The cathedral is open for guided tours and unguided visitations.

The castle was at its peak in the mid-16th century, during the reign of Duke John of Finland and Catherine Jagellon. They constructed the Renaissance Floor and the King's and Queen's hall, along with some other new features.

In 1573 to 1577, the deposed Swedish Queen Karin Månsdotter was kept prisoner there.

In 1614, a great fire destroyed the wooden structures of the old parts of the castle almost completely. King Gustav II Adolf was at the castle for a visit. After the fire, the main castle structure was largely abandoned and used as storage.

In the 18th century, the Renaissance bailey functioned as an administrative center, with the old main structure abandoned and in disrepair. The castle housed the provincial government after a period of turmoil called the Greater Wrath. The castle was used as a prison from the 18th to the end of the 19th century.