With a length of approximately 35 km (22 statute miles), Mariager Fjord is the longest fjord in Denmark. Mariager Fjord cuts into the Jutland peninsula from the Kattegat sea and ends at the town of Hobro; other important towns along the fjord are Hadsund and Mariager from which the fjord takes its name. Mariager fjord makes up most of the southern limit of the traditional region of Himmerland.
The width of Mariager Fjord varies from 4½ km to 250 metres (2,8-0,16 miles) and its area is about 47 km2s (18 square miles). The depth is up to 30 metres (ca. 100 ft).
As a result of the 2007 Municipal Reform, the new Mariagerfjord municipality was created around Mariager Fjord by fusion of four former municipalities.