Odorheiu Secuiesc (Romanian pronunciation: [odorˌheju sekuˈjesk]; Hungarian: Székelyudvarhely, Hungarian pronunciation: [ˈseːkɛjudvɒrhɛj] ^ⓘ; German: Oderhellen) is the second largest municipality in Harghita County, Transylvania, Romania. In its short form, it is also known as Odorhei in Romanian and Udvarhely in Hungarian. The Hungarian name of the town "Udvarhely" means "courtyard place".

At the 2011 census, the city had a population of 34,257; among those for whom data were available, 95.8% were ethnic Hungarians, making it the urban settlement with the third-highest proportion of Hungarians in Romania. The city was also home to communities of ethnic Romanians (2.6%) and Roma (1.5%). According to the 2021 census, Odorheiu Secuiesc had a population of 31,335; of those, 87.29% were Hungarians, 2.3% Romanians, and 1.58% Roma.

As of 2011, half the population of the town professed Roman Catholicism (50.05%), while the remaining half was primarily divided between Hungarian Reformed (30.14%), Unitarian (14.71%), and Romanian Orthodox (2.54%) communities.

A Roman fortress was found under the ruins of a medieval fort in the centre of the town. A funeral inscription was also found placed by the veteran, Aelius Equester, former centurion, to his wife Aurelia Juiuni and himself. The inscription dates back to the 7th century AD. Stamps with the letters of the Alpinorum equitata cohort and the Ubiorum cohort were also found in Odorhei. The town, as the former seat of the Udvarhely comitatus, is one of the historical centers of Székely Land. The first known reference to the city was in a papal register of duties in 1334 when it was mentioned by its Hungarian name, a sacerdos de Oduorhel. Since 1615, when Gabriel Bethlen, Prince of Transylvania, reaffirmed the rights of the town, the place has been referred to as Székelyudvarhely.

Udvarhely was the location of the first assembly of Székelys in 1357. A fortress was built in the town in 1451. It was rebuilt and strengthened by John II Sigismund Zápolya in 1565, in order to control the Székelys. Wallachian Prince Michael the Brave allied with the Szekelys and the Habsburgs destroyed the fortress in 1599 during their campaign in Transylvania. It was repeatedly rebuilt and destroyed again during history. The ruins of the structure are nowadays known as "The Székely-Attacked Fortress".

The town was historically part of the Szeklerland region of Transylvania. It was the seat of Udvarhelyszék District until the administrative reform of Transylvania in 1876, when it fell within the Udvarhely County in the Kingdom of Hungary. In the aftermath of World War I, the Union of Transylvania with Romania was declared in December 1918. At the start of the Hungarian–Romanian War of 1918–1919, the town passed under Romanian administration. After the Treaty of Trianon of 1920, it became part of the Kingdom of Romania and was the seat of Odorhei County during the interwar period. In 1940, the Second Vienna Award granted Northern Transylvania to Hungary. Towards the end of World War II, Romanian and Soviet armies entered the town in September 1944. The territory of Northern Transylvania remained under Soviet military administration until 9 March 1945, after which it became again part of Romania. In 1950, after Communist Romania was established, the city became the headquarters of the Odorhei Raion of Stalin Region. Between 1952 and 1960, the town fell within the Magyar Autonomous Region, between 1960 and 1968 the Mureș-Magyar Autonomous Region. After the administrative reform of 1968, the region was abolished, and since then, the town has been part of Harghita County.

The town and the surrounding villages were hit by a significant flood in August 2005.

A statue park of historical persons of importance for the Székelys was unveiled in the town on 22 May 2004. This gave rise to controversy, as one of the statues (The Wandering Szekler) was interpreted in the Romanian press as being the portrait of controversial writer and poet Albert Wass.