Dacia (/ˈdeɪʃə/, DAY-shə; Latin: [ˈd̪aː.ki.a]; Ancient Greek: Δακία) was the land inhabited by the Dacians, its core in Transylvania, stretching to the Danube in the south, the Black Sea in the east, and the Tisza in the west. The Carpathian Mountains were located in the middle of Dacia. It thus roughly corresponds to present-day Romania, as well as parts of Moldova, Bulgaria, Serbia, Hungary, Slovakia, Czech Republic, Poland and Ukraine.

A Dacian kingdom that united the Dacians and the Getae was formed under the rule of Burebista in 82 BC and lasted until the Roman conquest in AD 106. As a result of the wars with the Roman Empire, after the conquest of Dacia, the population was dispersed, and the capital city, Sarmizegetusa Regia, was destroyed by the Romans. However, the Romans built a settlement bearing the same name, Ulpia Traiana Sarmizegetuza, 40 km away, to serve as the capital of the newly established Roman province of Dacia. A group of "Free Dacians" may have remained outside the Roman Empire in the territory of modern-day Northern Romania until the start of the Migration Period.

The Dacians are first mentioned in the writings of the Ancient Greeks, in Herodotus (Histories Book IV XCIII: "[Getae] the noblest as well as the most just of all the Thracian tribes") and Thucydides (Peloponnesian Wars, Book II: "[Getae] border on the Scythians and are armed in the same manner, being all mounted archers"). Some historians argue that Daxia (mentioned in 3rd century BC) was the previous home of Indo-Iranian nomads who later came to form the Geto-Dacian people.

The extent and location of Dacia varied in its three distinct historical periods (see below):

The Dacia of King Burebista (82–44 BC) stretched from the Black Sea to the river Tisza. During that period, the Getae and Dacians conquered a wider territory and Dacia extended from the Middle Danube to the Black Sea littoral (between Apollonia and Pontic Olbia) and from the Northern Carpathians to the Balkan Mountains. After Burebista's death in 44 BC, Dacia plunged into internal strife, resembling a civil war, as his unified kingdom split into several rival states. The constant power struggle weakened Dacia, but the Dacians remained a significant force, frequently making incursions into Roman territory. Stability was only restored when Duras and later Decebalus managed to reunite the kingdom.

Strabo, in his Geography written around AD 20, says:

As for the southern part of Germany beyond the Albis, the portion which is just contiguous to that river is occupied by the Suevi; then immediately adjoining this is the land of the Getae, which, though narrow at first, stretching as it does along the Ister [Danube] on its southern side and on the opposite side along the mountain-side of the Hercynian Forest (for the land of the Getae also embraces a part of the mountains), afterwards broadens out towards the north as far as the Tyregetae; but I cannot tell the precise boundaries

On this basis, Lengyel and Radan (1980), Hoddinott (1981) and Mountain (1998) consider that the Geto-Dacians inhabited both sides of the Tisza river prior to the rise of the Celtic Boii. The hold of the Dacians between the Danube and the Tisza was tenuous. However, the archaeologist Parducz argued for a Dacian presence west of the Tisa dating from the time of Burebista. According to Tacitus (AD 56–117) Dacians bordered Germania in the south-east, while Sarmatians bordered it in the east.Although Strabo and Caesar notes that the Dacians reached the Hercynian Forest, Germania and beyond, Strabo still indicates their borders as the middle Danube and the northwestern Carpathians. This suggests that, while they exerted influence over more distant regions, they did not formally annex them. This also helps explain why some maps depict regions such as Bohemia or Pannonia as falling under Dacian territory or influence. However, most academic maps concentrate on formally annexed areas, as regions of temporary control or mere influence are inherently imprecise and difficult to define.