As of January 1, 2009 the city had a population of 145,151 inhabitants, making it the 16th largest city in Romania.
The city was first documented in 1332 in the papal registry under the name Novum Forum Siculorum, and as Sekulvasarhel (Székelyvásárhely) in 1349. On the place of its Castle Church, the Dominican's church stood until the Mongol invasion, when it was destroyed. In its place, the Franciscans built a new Gothic church in 1260 , which was completed in 1446. Since 1439 the town was the scene of the session of parliament ( diet) 36 times.In 1405, the King of Hungary Sigismund of Luxembourg granted the city the right to organize fairs. In 1470 King Matthias Corvinus granted the first judicial privilege to the city, and in 1482 declared the city a royal settlement. In 1492, wayvoda István Báthory strengthened its monastery with fortifications, this was a pentagon-shaped outer castle tower. In 1506, the troops of Pál Tomori were beaten by the Szeklers rising against the payment of an extraordinary Ox tax imposed on them on occasion of the birth of Louis II of Hungary. In 1557, the Reformed Church College (i.e. Presbyterians) was established as the oldest Hungarian school of Transylvania. In 1571, the session of Transylvanian parliament under prince John II Sigismund Zápolya accepted the free preach of the word of God, including the Unitarian Church. In 1600-1601, as a result of the siege of Giorgio Basta, the fortress turned to ruins. In 1602, the troops of Gergely Németh put on fire the remaining houses of the town, therefore, in 1602 the reconstruction of the fortress was started further the advice of mayor Tamás Borsos, but it was actually built between 1614 and 1653. Mózes Székely the only prince of Szekler origin visited the city in 1603, when liberated Transylvania from foreign domination. In 1616, it was granted the status of a free royal city under the name of Maros-Vásárhely by prince (fejedelem) Gábor Bethlen . In 1658, Turkish and Tartarian troops invaded and burned it, 3000 people were taken into captivity. In 1661, as no one show willingness to accept the duty of prince, under pressure from pasha Ali, Mihály Apafi was elected prince here. In 1662, resulting from the negligence of the Turkish military residing here, the city was almost completely burnt down. In 1687, it was devastated by German imperial troops.
In 1704, the kuruc troops of Pál Kaszás occupied the fortress, which was re-occupied by Lőrinc Pekry from the labanc in 1706. On 5 April 1707, Francis II Rákóczi was raised to the chair of princes. In 1707 it was struck by pest, more than 3500 people died, the black death renewed in 1709, 1719 and in 1738-39.
The city received a major boost to its social and economic life when it became home to supreme court of justice of the Principality of Transylvania in 1754. In 1802, the Teleki Library founded by count Sámuel Teleki was opened for the public with 40.000 volumes.
Avram Iancu, the leader of the 1848 Romanian revolution in Transylvania, was a young lawyer in the city of Marosvásárhely before engaging in the fight for the rights of Romanians living in Transylvania. On 4 November 1848, the Szekler troops were beaten by the Austrian imperial troops under its walls, and the city was also captured. On January 13, 1849 the troop of major Tolnay recaptured it. On 30 July 1849, Sándor Petőfi and Bem set out from here for the Battle of Segesvár.
In 1854, Szekler martyrs Károly Horváth, János Török and Mihály Gálfi were executed on the Postarét for plotting against the Austrian rule, since 1874 a monument marks the place. In 1861, Marosvásárhely became the seat of Marosszék, in 1876 that of Maros-Torda County. In 1880 the statue of Bem was inaugurated in Roses Square, in downtown area; in 1893 the statue of Kossuth was as well. The statue of Rákóczi was also inaugurated in 1907. All three were demolished after World War I between 1919 in 1923 after Transylvania became part of Romania.
The provincial appearance of the city changed greatly in the late 19th century and early 20th century. In 1913, the Hungarian Art-Nouveau style city hall complex and Cultural Palace was opened, as part of mayor Bernády György's urban renewal. After World War I, together with the rest of Transylvania, Marosvásáshely became part of Romania and was re-named Oşorheiu. From having been an 89% Hungarian-populated city (1910), Romanian population increased throughout the latter half of the 20th century.
From 1940 to 1944, as a consequence of the Second Vienna Award, the city was ceded back to Hungary. After Hungary was occupied by Germany in 1944, a Jewish ghetto was established in the city. Oşorheiu re-entered the Romanian administration at the end of the war in October 1944, however, on 12 November 1944 general Vinogradov of the Soviet Red Army expulsed the returning Romanian authorities from Northern Transylvania with reference to the massacres committed by members of Iuliu Maniu's so called Maniu-guard, and the Romanian authorities were not allowed to return until the government of Petru Groza was formed on 6 March 1945.
After World War II, the communist administration of Romania conducted a policy of massive industrialization that completely re-shaped the community. Between 1950-1968, it was the center of the Hungarian Autonomous Province, later named as Mures-Hungarian Autonomous Region. On 7 September 1959, Gheorghe Gheorghiu-Dej, Secretary-General of the Romanian Workers Party, and the Prime Minister Chivu Stoica visited the city. It was then decided where to build the fertilizer production plant, and the new residential quarters of the city. It was decided that the residential quarters would not be built in the Maros valley, but on the surrounding hills.
In March 1990, shortly after the Romanian Revolution of 1989 overthrew the communist regime, the city was the stage of violent confrontations between ethnic Hungarians and ethnic Romanians .
As of 2000, a considerable percentage of its population has started to work abroad temporarily. The local economy has started to get stronger after various investors settled in the area.
The city has a substantial ethnic Hungarian minority, some of whom identify as Székelys. Since 2003 some Székely organizations have been campaigning for the city to become the center of an autonomous region again.
