Nidaros Cathedral in the context of "1905 Norwegian monarchy referendum"

Trondheim (UK: /ˈtrɒn(d)haɪm/ TRON(D)-hyme, US: /ˈtrɒnheɪm/ TRON-haym; Urban East Norwegian: [ˈtrɔ̂nː(h)æɪm]; Southern Sami: Tråante), historically Kaupangen, Nidaros, and Trondhjem (locally [ˈtrɔ̂nːjæm] ), is a city and municipality in Trøndelag county, Norway. As of 2025, it had a population of 216,518. Trondheim is the third most populous municipality in Norway, and is the fourth largest urban area. Trondheim lies on the south shore of Trondheim Fjord at the mouth of the River Nidelva. Among the significant technology-oriented institutions headquartered in Trondheim are the Norwegian University of Science and Technology (NTNU), the Foundation for Scientific and Industrial Research (SINTEF), the Geological Survey of Norway (NGU), and St. Olavs University Hospital.

The settlement was founded in 997 as a trading post and served as the capital of Norway through the Viking Age until 1217. From 1152 to 1537, the city was the seat of the Catholic Archdiocese of Nidaros; it then became, and has remained, the seat of the Lutheran Diocese of Nidaros and the site of the Nidaros Cathedral. The city was incorporated in 1838. The current municipality was formed in 1964, when Trondheim merged with Byneset Municipality, Leinstrand Municipality, Strinda Municipality, and Tiller Municipality, and further expanded on 1 January 2020, when Trondheim merged with Klæbu Municipality.

Saint Olaf (c. 995 – 29 July 1030), also called Olaf the Holy, Olaf II, Olaf Haraldsson, and Olaf the Stout or "Large", was King of Norway from 1015 to 1028. Son of Harald Grenske, a petty king in Vestfold, Norway, he was posthumously given the title Rex Perpetuus Norvegiae (English: Eternal/Perpetual King of Norway) and canonised at Nidaros (Trondheim) by Bishop Grimketel, one year after his death in the Battle of Stiklestad on 29 July 1030. His remains were enshrined in Nidaros Cathedral, built over his burial site. His sainthood encouraged the widespread adoption of Christianity by Scandinavia's Vikings/Norsemen.

Pope Alexander III confirmed Olaf's local canonisation in 1164, making him a recognised saint of the Catholic Church, and Olaf started to be known as Rex Perpetuus Norvegiae – eternal king of Norway. Following the Reformation, he was a commemorated historical figure among some members of the Lutheran and Anglican Communions.

Nidaros, Niðarós or Niðaróss (Old Norse pronunciation: [ˈniðɑˌroːsː]) was the medieval name of Trondheim when it was the capital of Norway's first Christian kings. It was named for its position at the mouth (Old Norse: óss) of the River Nid (the present-day Nidelva).

Although the capital was later moved to Oslo (around the year 1300), Nidaros remained the centre of Norway's spiritual life until the Protestant Reformation. The Archdiocese of Nidaros was separated from Lund (in Scania) by the papal legate Nicholas Breakspeare in 1152, and the shrine to Saint Olaf in Nidaros Cathedral was Northern Europe's most important pilgrimage site during the Middle Ages. Archbishop Olav Engelbrektsson led Norway in its attempted resistance against the Danish Reformation, and was forced into exile by King Christian III in 1537. The archdiocese was abolished and replaced with a Lutheran diocese.

The Archdiocese of Nidaros (or Niðaróss) was the metropolitan see covering Norway in the later Middle Ages. The see was the Nidaros Cathedral, in the city of Nidaros (now Trondheim). The archdiocese existed from the middle of the twelfth century until the Protestant Reformation.

Nidaros is a diocese in the Lutheran Church of Norway. It covers Trøndelag county in Central Norway and its cathedral city is Trondheim, which houses the well-known Nidaros Cathedral. Since 10 September 2017, the Bishop of Nidaros is Herborg Finnset. The Bishop Preses, currently Olav Fykse Tveit is also based at the Nidaros Cathedral. The diocese is divided into nine deaneries (prosti). While the Bishop Preses holds episcopal responsibility within the Nidaros domprosti (deanery) in Trondheim, the Bishop of Nidaros holds episcopal authority of the other eight deaneries as well as the language based parish of the Southern Sámi.