The Tumen River (Chinese: 图们江; pinyin: Túmén Jiāng, Russian: река Туманная, Korean: 두만강; Korean pronunciation: [tumaŋgaŋ]), also known as the Tuman River or Duman River, is a 521-kilometre (324 mi) long river that serves as part of the boundary between China (left shore), North Korea (right) and Russia (left), rising on the slopes of Mount Paektu and flowing into the Sea of Japan. The river has a drainage basin of 33,800 km² (13,100 sq mi).

The river flows in northeast Asia, on the border between China and North Korea in its upper reaches, and between North Korea and Russia in its last 17 kilometers (11 mi) before entering the Sea of Japan. The river forms much of the southern border of Jilin Province in Northeast China and the northern borders of North Korea's North Hamgyong and Ryanggang provinces. Paektu Mountain on the Chinese-North Korean border is the source of the river, as well as of the Yalu River. The two rivers and the region of Paektu Mountain between their headwaters form the border between North Korea and China.

The name of the river comes from the Mongolian word tümen, meaning "ten thousand" or a myriad. In Tumen, Jilin, a riverfront promenade has restaurants where patrons can gaze across the river into North Korea. The Russian name of the river is Tumannaya, literally meaning foggy.

In 1938 the Japanese built the Tumen River Bridge, where the Quan River meets the Tumen River, between the villages of Wonjong (Hunchun) and Quanhe. Important cities and towns on the river are Hoeryong and Onsong in North Korea, Tumen and Nanping (南坪镇, in the county-level city of Helong) in China's Jilin province.

In 1995, the People's Republic of China, Mongolia, Russia, North Korea and South Korea signed three agreements to create the Tumen River Economic Development Area.

Noktundo, a former island (now effectively a peninsula) at the mouth of the Tumen, has been a boundary contention between Russia and North Korea. The Qing dynasty ceded the island to Russia as part of the Primorsky Maritimes (East Tartary) in the 1860 Treaty of Peking. In 1990, the then Soviet Union and North Korea signed a border treaty which made the border run through the center of the river, leaving territory of the former island on Russian side. South Korea refuses to acknowledge the treaty and demanded that Russia return the territory to Korea.

There are several popular species of fish endemic to Tumen river, such as Tumen lenok and bighead gudgeon. In 2016, China released 800,000 salmon seedlings into Tumen river in order to expand the regional fishing industry and meet the increasing demand for sea products.

The Tumen has been crossed for years by North Korean refugees defecting across the Chinese border. Most refugees from North Korea during the 1990s famine crossed it, and most recent refugees have also used it, as it is far easier than crossing the Amnok River. The Tumen is the preferred crossing because, unlike the swift, deep and broad Amnok River that runs along most of the border between the two countries, the Tumen is shallow and narrow. In some areas it can be crossed on foot, or by short swims. It freezes in winter, allowing dry crossings. Defectors wishing to cross the Tumen often ignore its pollutants and dangerous border patrol, and spend weeks if not months or years waiting for the perfect opportunity to cross. "Long, desolate stretches of the Chinese-North Korean border are not patrolled at all", according to a New York Times article.