Arad County was an administrative unit in the Kingdom of Hungary, the Eastern Hungarian Kingdom and the Principality of Transylvania. The county was established along the Maros (Mureș) River in the 11th or the 12th century, but its first head, or ispán, was only mentioned in 1214. Its territory is now part of Romania, except a small area (the town of Elek and the surrounding villages) which is part of Hungary. The capital of the county was Arad.

The medieval Arad County was situated in the lands along both banks of the Maros (Mureș) River. The existence of arable lands, pastures, vineyards and orchards in the western lowlands in the Middle Ages is well-documented. The hilly eastern regions were sparsely populated. The total territory of the medieval county was around 3,800 km² (1,500 sq mi).

In 1744, Arad County absorbed a large part of Zaránd County, including its capital Zaránd/Zărand (the remainder of Zaránd County was then reorganized, with Körösbánya/Baia de Criș as the new capital). In 1876, when the administrative structure of the Kingdom of Hungary was changed, the territory of Arad County was further modified to include the western third of the diminished Zaránd County, namely the Nagyhalmágy/Hălmagiu district (Zaránd County was disbanded altogether on that occasion).

Arad County shared borders with the Hungarian counties of Csanád, Békés, Bihar, Torda-Aranyos, Hunyad, Krassó-Szörény, Temes, and Torontál. The river Mureș formed its southern border. Crișul Alb River flowed through the county. Its area was 6,078 km² (2,347 sq mi) around 1910.

The Hungarians dominated the region of the Maros in the middle of the 10th century, according to the Byzantine Emperor Constantine Porphyrogenitus. Archaeological finds also shows that Hungarians settled in the plains along the river after their arrival in the Carpathian Basin at the end of the 9th century. Place names of Slavic origin, including Lipova (Lippa) and Zăbrani (Temeshidegkút), evidence the presence of Slavic speaking communities, especially in the region where the river, coming from the mountains, reached the lowlands.

A powerful chieftain, Ajtony, ruled the territory along the rivers Danube, Maros and Tisza in the early 11th century. The Maros formed the northern border of Ajtony's realm, according to the Gesta Hungarorum, but the longer version of the Legend of Saint Gerard wrote that he controlled the lands as far as the Körös River. Ajtony was killed in a battle against the army of Stephen I of Hungary, which was under the command of Csanád. According to a scholarly theory, first proposed by historian György Györffy, Stephen I established Arad County after Ajtony's fall. On the other hand, historian Gyula Kristó writes that Ajtony's whole realm was transformed into the large Csanád County during Stephen I's reign; Arad County only developed into a separate administrative unit in the second half of the 11th century or in the 12th century.

The remains of an 11th-century stronghold, made of earth and timber, were found at Arad. At an assembly held in Arad in 1131, the wife of King Béla the Blind, Helena of Rascia, ordered the massacre of 68 Hungarian lords. Arad Castle and the estates attached to it were first documented in a royal charter, issued in 1177. The first known ispán, or head, of Arad County, Paul Csanád, was mentioned in a royal diploma, dated to 1214, but its authenticity is suspect. The earliest authentic document that referred to an ispán of Arad was issued in 1240. The western regions of the county were included in the Deanery of Arad of the Roman Catholic Diocese of Csanád; the Archdean of Arad was first mentioned in 1288.

The earliest documents suggest that the kings owned most lands in the plains along the Maros. However, the distribution of royal domains started at the end of the 11th century. For instance, Ladislaus I of Hungary and his brother, Lampert, granted five villages to the Titel Chapter before 1095 and Béla the Blind established the Arad Chapter in the 1130s. The Hodos clan was the only local noble kindred in the county; they were the patrons of the Hodoș-Bodrog Monastery. Members of the Csanád, Csák and Dorozsma clans received estates in Arad County in the early 13th century.