Trondheim, historically Kaupangen, Nidaros and Trondhjem, is a city and municipality in Trøndelag county, Norway. As of 2022, it had a population of 212,660. Trondheim is the third most populous municipality in Norway, and was 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.

Nidaros, Niðarós or Niðaróss 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 of the River Nid.