Namsen (Norwegian) or Nååmesje (Southern Sami) is one of the longest rivers in Trøndelag county, in the central part of Norway. The 228-kilometre (142 mi) long river flows through the municipalities of Røyrvik, Namsskogan, Grong, Overhalla, and Namsos before emptying into the Namsenfjorden. The river is the namesake for the whole Namdalen region. The river traditionally has been used for floating timber down from the forests to the town of Namsos, where the sawmills were located. Today, parts of the Namsen are regulated by several dams.

The river begins in springs in Børgefjell National Park, just over the border in Nordland county. This water feeds the large lake Namsvatnet. The Namsen river itself starts when the water passes through the dam on the northwest side of the lake Namsvatnet in Røyrvik Municipality.

The river then travels through the Namdalen valley towards the coast, ending at the town of Namsos where it flows into the Namsenfjorden, the same fjord into which the smaller river Årgårdselva flows. There are two major tributaries to the Namsen: Tunnsjøelva and Sanddøla.

The total watershed is approximately 6,298 square kilometres (2,432 sq mi) and at Namsos, the water discharges into the fjord at about 285 cubic metres per second (10,100 cu ft/s).

The Namsen is considered one of the best Atlantic Salmon fishing rivers in the world, and is often called the "Queen of Rivers". Beginning in the 19th century, the British found the Atlantic salmon fishing to be excellent here, and the river became a major tourist attraction. Catches above 23 kilograms (51 lb) are not unusual. The Namsen is a wide river and is thus often fished from small boats using a method called "harling". This method consists of trolling a lure while the boat moves slowly from bank to bank and drifting downstream. Thus, the salmon meet the lure as they swim upstream.

• Kraftverkene i Øvre Namsen
• The Norwegian Sawmill museum

• Salmon with a weight of 25 kg caught in June 2009 (in Norwegian)
• Upper Namsen Fishing