Mariagerfjord is a municipality (Danish, kommune) in North Denmark Region in Denmark. It covers an area of 723.63 km² (2011) and a has a population of 42,093 (2014).
On January 1, 2007 Mariagerfjord municipality was created as the result of Kommunalreformen ("The Municipal Reform" of 2007), consisting of the former municipalities of Arden, Hadsund and Hobro, and the major part of Mariager, and a very small part of Aalestrup and a very small part of Nørager municipality.
The municipality consists of the land around Mariager Fjord, its namesake.
Coordinates: 56°41′N 9°56′E / 56.683°N 9.933°E