Nidaros was the old name of Trondheim, Norway, in the middle ages. Nidaros was Northern Europe's most important Christian pilgrimage site during this time, the pilgrims' goal being the Nidaros Cathedral, established as the seat of the archdiocese in 1152 by Nicholas Breakspeare, who later became the only English pope as Adrian IV. The town owed its name to the location at the mouth of the Nidelva river (os = river mouth).

Nidaros is also the name of the Medieval Catholic Metropolitan Archdiocese of Norway, which had suffragans in present-day Norway, Iceland, Greenland, Faroe Islands, Shetland Islands, Scotland and the Isle of Man.

After the Catholic Archdiocese was abolished at the Protestant Reformation in 1537, a Lutheran superintendanture, now diocese, with the name of Nidaros was erected in its place.

The city of Trondhjem actually changed name back to Nidaros on January 1 1930. After a fierce campaign among the citizens against the new name, the Norwegian Parliament changed its name to Trondheim on March 6 1931.