Nanaimo River is a river on Vancouver Island, British Columbia, Canada, located near the city of Nanaimo on the island's east coast. Its headwaters are in the Vancouver Island Ranges of central Vancouver Island and its mouth, the Nanaimo River estuary, is at the south end of Nanaimo Harbour in the Strait of Georgia. The estuary is part of the Pacific Estuary Conservation Program.

The river and its drainage basin contain a wide range of industrial and recreational activities. Logging and coal mining were a couple of the earliest industrial operations. While coal mining ceased in the first half of the 20th century, logging continues to be important. Morden Colliery Historic Provincial Park is one of the few places left near the river that links to the region's coal mining history. Due to the lack of arable land, agriculture never became an important industry. In 1931, South Fork Dam was built upstream on the South Nanaimo River to supply water to the city of Nanaimo. In 1974, an additional dam and reservoir were built farther upstream on a tributary of the South Nanaimo River to increase the water supply for the city. In 2000, the South Fork Dam was upgraded to produce Hydroelectricity.

Two major highways cross the river. The Trans-Canada Highway crosses the river just north of Cassidy and farther downstream Highway 19 crosses on the north side of Cedar. At the point where the Trans-Canada Highway crosses the river, the Esquimalt & Nanaimo Railway also crosses it. On the east side of Cassidy, just of the south bank of a major tributary of the river, Haslam Creek, is the Nanaimo Airport

All along the river, and throughout its watershed, there are plenty of lakes and recreational opportunities, including boating, camping, hiking, bungee jumping, and rock climbing. There are still pockets of old growth forest left within easy access to the public, such as at the Nanaimo River Regional Park.

Along the Nanaimo River are the archaeological remains of five fall and winter Coast Salish villages. The villages are called Solachwan, Tewahlchin, Anuweenis, Kwalsiarwahl, and Ishihan. The villages were often occupied until April, in which the Coast Salish would then travel to Gabriola Island to hunt. The Nanaimo River was the lifeblood of the Coast Salish people living in modern-day Nanaimo.

The Nanaimo River is part of the traditional territory of the Snuneymuxw and Stz'uminus First Nations. In total, the Snuneymuxw have six Indian reserves in the Nanaimo area. Nanaimo Town 1 is located on the west side of Nanaimo Harbour, just south of downtown Nanaimo. Three others, Nanaimo River 2, 3, and 4 are located on the Nanaimo River delta. Gabriola Island 5 is on Gabriola Island and Ma-guala 6 is on a small island in Degnen Bay on the south-west coast of Gabriola Island.

The source of the Nanaimo River is the eastern slopes of Mount Hooper. Mount Hooper is on the drainage divide that separates rivers flowing eastward towards the Strait of Georgia and westwards into the open Pacific Ocean. The western slopes of Mount Hooper are in the Nitinat River's watershed.

From its source, the Nanaimo River flows east for most of its length then north into the south end of Nanaimo Harbour. There are two lakes, First and Second, along the river's course and multiple tributaries and other lakes in its watershed.