The Middle Ages in Romania began shortly after the withdrawal of the Roman legions from the former Roman province of Dacia in the late 3rd century and with the start of the Early Middle Ages and the Migration Period that followed afterwards respectively. It subsequently came to an end with the reign of Domn Michael the Brave (1593–1601) who managed, for a short time between 1599 and 1600, to rule Wallachia, Moldavia, and Transylvania together, the three principalities whose territories were to be united some three centuries later to form modern and contemporary Romania.

Over most of this period, Banat, Crișana, Maramureș, and Transylvania – now regions in Romania to the west of the Carpathian Mountains – were part of the Kingdom of Hungary. They were divided into several types of administrative units, such as "counties" and "seats". The heads of the Transylvanian counties or "counts" were subordinated to a special royal official called voivode, but the province was seldom treated as a single unit, since the Székely and Saxon seats were administered separately. In the kingdom, Romanian peasants, being Orthodox, were exempt from the tithe, an ecclesiastical tax payable by all Roman Catholic commoners. However, Romanian noblemen slowly lost the ability to participate in political life, as the 14th-century monarchs pursued a zealous pro-Catholic policy. Their position became even worse after 1437 when the so-called "Union of Three Nations", an alliance of the Hungarian noblemen, the Székelys, and the Saxons, was formed in order to crush the Bobâlna peasant uprising.

Wallachia, the first independent medieval state between the Carpathians and the lower Danube was created when Basarab I ( 1310–1352) terminated the suzerainty of the King of Hungary with his victory in the battle of Posada in 1330. The independence of Moldavia, to the east of the Carpathians, was achieved by Bogdan I (1359–1365), a nobleman from the Voivodeship of Maramureș, who led a revolt against the former ruler who was appointed by the Hungarian monarch. The independence of the two principalities, however, was rarely secure, and vassalage to multiple states became an important aspect of their diplomacy. Although Wallachia paid tribute to the Ottoman Empire from 1417, and Moldavia from 1456, their two medieval monarchs, Mircea the Old of Wallachia (1386–1418) and Stephen the Great of Moldavia (1457–1504) conducted successful military operations against the Ottoman Turks. The two principalities' trade with other parts of Europe began to decrease after the last decades of the 15th century. Before this the sale of hides, grain, honey, and wax to the Holy Roman Empire, Venice, and Poland, and the import of silk, weapons and other manufactured goods from these areas had been widespread, but by the end of the 16th century the Ottoman Empire became the main market for Romanian products.

Transylvania, together with the neighboring counties, gained the status of an autonomous state under Ottoman suzerainty after the central territories of the Kingdom of Hungary had been annexed by the Ottomans in 1541. The fall of the kingdom also deprived Wallachia and Moldavia of their main ally in the struggle against the Ottoman Empire. In 1594, Michael the Brave of Wallachia joined the anti-Ottoman alliance initiated by Pope Clement VIII. After a series of victory over the Ottomans, he turned against Transylvania and Moldavia where pro-Polish and pro-Ottoman princes were reigning. He invaded and occupied Transylvania in 1599, and Moldavia in 1600. Although the personal union of the three countries collapsed four months later, it served as an ideal for later generations working for the unification of the lands that now form Romania.

Early Middle Ages (late 8th century–early 13th century)

At the end of the 8th century, the establishment of the Khazar Khaganate north of the Caucasus Mountains created an obstacle in the path of nomadic people moving westward.Treptow et al. 1997, p. 52.Spinei 2009, pp. 47–49. In the following period, the local population of the Carpathian–Danubian area profited from the peaceful political climate and a unitary material culture, called "Dridu", that developed in the region.Spinei 2009, p. 50. Finds from the Dridu settlements, such as coulters and sickles, confirm the role of agriculture in their economy.Spinei 2009, p. 223.

In the 9th century centrifugal movements commenced in the Khazar Kaghanate.Klepper 2002, p. 44. One of the subject peoples, the Hungarians, left the Khanate and settled in the region between the rivers Don and Dniester. They abandoned the steppes and crossed the Carpathians around 896. According to the 13th-century Gesta Hungarorum ("Deeds of the Hungarians") at the time of the Hungarian conquest, Transylvania was inhabited by "Vlachs and Slavs" (blasij et sclaui) and ruled by Gelou, "a certain Vlach" (quidam blacus), while Crișana was inhabited by several peoples, among them Székelys. Whether the author of the Gesta had any knowledge of the real conditions of the turn of the 9th and 10th centuries remains debated by historians.Spinei 2009, p. 75.

In 953, the gyula, the second leader in rank of the Hungarian tribal federation, converted to Christianity in Constantinople. Around that time, according to the Byzantine Emperor Constantine VII, the Hungarians controlled the region on the border of modern Romania and Hungary along the rivers Timiș, Mureș, Criș, Tisa and Toutis. In 1003, as the Annals of Hildesheim narrates, Stephen I, the first crowned monarch of Hungary (c. 1000–1038) "led an army against his maternal uncle, Gyula, and occupied Gyula's country.

Stephen I granted privileges to the Roman Catholic Church, for example by ordering the general imposition of the tithe upon the population.Sedlar 1994, p. 164. Burials in most local pre-Christian cemeteries, for example at Hunedoara, only ceased around 1100. Stephen I also divided his kingdom, including the territories of modern Romania he had occupied, into counties, that is administrative units around royal fortresses, each administered by a royal official called count.Sedlar 1994, p. 275. In time the voivode, a higher royal official first attested in 1176, became the principal of all the counts in Transylvania. In contrast with Transylvania, the counts in modern Banat and Crișana remained in direct contact with the king who appointed and replaced them at will.

From the end of the 9th century the Pechenegs controlled the territories to the east and south of the Carpathians. According to the Eymund's saga, they fought together with the Blökkumen ("Romanians") in the Kievan Rus' in the 1010s.Spinei 2006, pp. 103–105. The Pechenegs were swept aside from their territories by the Cumans between 1064 and 1078.Klepper 2005, p. 46. A late variant of the oldest Turkish chronicle, the Oghuz-name relates that the Cumans defeated many nations, including the Ulâq ("Romanians"). Some of the Pechenegs fled into the Kingdom of Hungary where they were employed to guard the border districts, for instance in Transylvania.Sedlar 1994, p. 11.

The 11th-century settlements in Transylvania are characterized by small huts with ceramic assemblages marked by clay cauldrons. The increasing number of coin finds suggests that the province experienced economic growth in the late 11th century. The first document pertaining to the province is a royal charter of 1075 referring to taxes on salt levied at Turda. The earliest precious metal mine in medieval Transylvania, the silver mine at Rodna was first mentioned in 1235.

In the 12th and 13th centuries hospites ("guest settlers") arrived in Transylvania from Germany and from the French-speaking regions on the river Rhine who in time became collectively known as "Saxons".Treptow et al. 1997, pp. 62–63. In 1224 Andrew II of Hungary (1205–1235) granted special liberties to the Saxons who had settled in southern Transylvania. For instance, they were authorized to choose their local leaders; only the head of the entire community, the count of Sibiu, was appointed by the king. They were also granted the right to use "the forest of the Romanians and the Pechenegs". The first references to viniculture in Transylvania are connected to the vineyards of the hospites of Cricău, Ighiu, and Romos.

As a result of the Saxon immigration, the Székelys – Hungarian-speaking free warriors cultivating communal lands – were transferred to the southeast of the province.Treptow, Popa 1996, p. 194.Sedlar 1994, pp. 92–93. From the 13th century they were governed independently from the voivode by a royal official, the count of the Székelys. Besides the Saxons, the Cistercians became the agents of expansion in Transylvania. When their abbey at Cârța was established in the early 13th century, Andrew II ordered that the strip of land running up to the mountains between the rivers Olt, Cârțișoara and Arpaș be transferred from the Romanians to the new monastery.

Following the defeat of the Cumans by the Mongols on the river Kalka in 1223, some chieftains of the western Cuman tribes accepted the authority of the king of Hungary.Sedlar 1994, p. 211. Their conversion led to the creation of the Roman Catholic Diocese of Cumania to the east of the Carpathians.Spinei 2006, p. 154. However, the Orthodox Romanian population of the territory received the sacraments from "some pseudo-bishops of the Greek rite", according to a papal bull of 1234.Spinei 2006, pp. 154–155. In 1233 Oltenia was organized into a military frontier zone of the Kingdom of Hungary, called the Banate of Severin.

The expansion across the Carpathians was stopped by the invasion of the Mongols that lasted from March 31, 1241 to April, 1242. It was a major watershed in the medieval history of the region: although the number of casualties is disputed, even the most prudent estimates do not go below 15 percent of the total population.

High Middle Ages (1242–1396)

Outer-Carpathian regions

After the withdrawal from the Kingdom of Hungary, the Mongol forces halted at Sarai (now Russia) on the Volga River where their leader, Batu Khan set up his own capital.Sedlar 1994, p. 214. Henceforth, the steppes between the rivers Dnieper and Danube were under the influence of the Mongols of the Volga, known as the Golden Horde.Sedlar 1994, p. 215.Vásáry 2005, p. 69. From the 1260s, a relative of Batu, Nogai Khan settled at Isaccea on the Lower Danube and became the absolute master of the neighboring regions.Vásáry 2005, pp. 71–72. He made himself independent of the Golden Horde around 1280, but was killed in a battle in 1299.Vásáry 2005, pp. 91–92, 161.

By the middle of the 14th century, the westernmost Mongol territories had become subject to frequent Polish and Hungarian military offensives. Grand Prince Olgierd of Lithuania penetrated farther into the territories controlled by the Golden Horde than any European army had hitherto done.Sedlar 1994, p. 380. He won a major victory over the united Mongol troops on the Dnieper near the Black Sea in 1363.Vásáry 2005, p. 164.

Intra-Carpathian regions

Having been raided twice by the Mongols within a single year, Transylvania felt the consequences of the invasion of 1241–1242 for more than two decades. The administrative centers of the province, such as Alba Iulia and Cetatea de Baltă, had been destroyed. Due to the severe depopulation, a process of organized colonization commenced that lasted for several decades. For example, a new wave of colonization resulted in the establishment of the Saxon seats of Sighișoara and Mediaș; and the lord of Ilia received, in 1292, royal permission to settle Romanians in the lands he owned.