Austrasia was a historical region and the northeastern realm within the core of the Frankish State during the Early Middle Ages, centering on the regions between Meuse, Moselle, Middle Rhine and the Main rivers. It included the original Frankish-ruled territories within what had been the northernmost part of Roman Gaul and parts of Roman Germania. It also stretched beyond the old Roman borders on the Rhine into Frankish areas which had never been formally under Roman rule. It came into being as a part of the Frankish Kingdom, founded by the Merovingian king Clovis I (r. 481–511), who expanded Frankish rule further to the southwest, into Gaul, whose northern regions came to be known as Neustria.

These two realms, or sub-kingdoms (Austrasia and Neustria), along with Aquitaine and Burgundy, were subsequently ruled by various rulers from the Merovingian dynasty, followed in the 8th and 9th centuries by their successors from the Carolingian dynasty, whose own powerbase was in Austrasia itself. The two Frankish dynasties did not always have a single ruling monarch over the whole Frankish realm, and already by 561, Austrasia was ruled as a separate kingdom within the Frankish realm by the Merovingian king Sigebert I (561–575). Kings often allowed different family members to rule sub-kingdoms, and these were sometimes in conflict with each other, despite the underlying continuity of the overall Frankish state.

In 843, by the Treaty of Verdun, Austrasia was divided in three parts, with eastern section being assigned to the East Francia, central regions to the Middle Francia, and the most western part to the West Francia. Further divisions affected mostly the central part of Austrasia, ruled since the Treaty of Prüm (855) by king Lothair II, whose name gave rise to Lotharingia, that corresponded to central Austrasia. It was divided by the Treaty of Meerssen (870), but reunited by the Treaty of Ribemont (880), under the east Frankish rule. During the 9th century, Austrasia was still considered as one of the main regions (stem lands) within the Carolingian Empire, surrounded by Alamannia (to the south), Bavaria (to the south-east), Thuringia (to the north-east), and Saxony (to the north), as attested by the Royal Frankish Annals, and the Annals of Saint Bertin, that mentions not only Austrasia, but also Autrasians.

In time, central Austrasia was more frequently designated as Lotharingia, that became the prevailing term for those regions, while eastern parts of Austrasia came to be known as Franconia.

The name Austrasia is not well attested in the Merovingian period. The first surviving record of the term is by Gregory of Tours, writing in about 580. It was later used by Aimoin of Fleury around 1000. It is presumably the latinization of an Old Frankish name, reconstructed as *Oster-rike ("Eastern Kingdom"). As with the name Austria, it contains the word for "east", and means "eastern land". The term designated the original territory of the Franks in contrast to Neustria, which apparently meant the "(new) western land".

Austrasia was centered on the Middle Rhine, including the basins of the Moselle, Main and Meuse rivers. It bordered on Frisia and Saxony to the north, Thuringia to the east, Swabia and Burgundy to the south and to Neustria to the southwest. The exact boundary between Merovingian Neustria and Austrasia is unclear with respect to areas such as the medieval counties of Flanders, Brabant and Hainaut, and areas immediately to the south of these.

Metz served as the Austrasian capital, although some Austrasian kings ruled from Reims, Trier and Cologne. Other important cities included Verdun, Worms and Speyer. Fulda monastery, an important royal monastery, was founded in eastern Austrasia in the final decade of the Merovingian period.

In the High Middle Ages, its territory became divided among the duchies of Lotharingia and Franconia in Germany, with some western portions including Reims and Rethel passing to France.