Bolshoi Ussuriysky Island (Russian: Большо́й Уссури́йский о́стров, romanized: Bol'shoy Ussuriyskiy Ostrov), or Heixiazi Island (simplified Chinese: 黑瞎子岛; traditional Chinese: 黑瞎子島; pinyin: Hēixiāzi Dǎo; lit. 'black bear island'), is a sedimentary island at the confluence of the Ussuri and Amur rivers. Since the Sino-Russian Border Agreement that was fully implemented in 2008, Bolshoy Ussuriysky Island was divided between China and Russia. China was given control over part of the island, and Russia retained the rest. Since then, the issue has been peacefully settled, and China no longer claims the entire island. It has an area of about 327 to 350 km and is bounded closely by Yinlong/Tarabarov Island, and over 90 islets. Its position at the confluence of the Amur and the Ussuri, and right next to the major Russian city of Khabarovsk, has given it great strategic importance.

The 1860 Convention of Peking stipulated that the boundary between Russia and China lay along the Amur and Ussuri rivers. As such, the island at the confluence of the two rivers was Chinese. Until 2004, Bolshoy Ussuriyskiy Island was the site of a territorial dispute between mainland China and Russia. The Soviet Union forcefully occupied Bolshoy Ussuriyskiy and Yinlong Islands in 1929 in the wake of a Russo-Manchurian conflict, but this was not accepted by China. While Russia governed the islands as a part of Khabarovsk Krai, China claimed them as a part of Fuyuan County, Heilongjiang province, the easternmost part of China.

The difficulty in settling this dispute involved competing interests between Russia and China. To settle the boundary along the lines claimed by China would have subjected settled parts of the city of Khabarovsk to the range of artillery emplaced on Heixiazi. However, by occupying the entire island, Russia controlled the entire Amur and Ussuri waterway and gave Khabarovsk a comfortable buffer zone. During its control of the island, Russia refused navigational access to the Amur and Ussuri to Chinese ships.

On October 14, 2004, the Complementary Agreement between the People's Republic of China and the Russian Federation on the Eastern Section of the China-Russia Boundary was signed, in which Russia agreed to relinquish control over Yinlong Island and around half of Bolshoy Ussuriysky. About 170 square kilometres of Bolshoy Ussuriysky was transferred to mainland China, while the rest remained under Russia's jurisdiction. In return, mainland China agreed to drop all territorial claims to the remainder of Bolshoy Ussuriysky kept by Russia and received the right to navigate ships along the main channel of the Amur.

In 2005, the Russian Federal Assembly and the mainland Chinese National People's Congress approved the Sino-Russian Border Agreement. On July 21, 2008, an agreement was signed in Beijing by the mainland Chinese and Russian Foreign Ministers that finalized the border demarcation and formally ended negotiations. Under the agreement, Russia would transfer approximately 174 km of territory to mainland China. The transfer took place on October 14, 2008. The area transferred to mainland China is largely uninhabited. The Mainland Chinese part of the island is situated in the district of Fuyuan City in the province of Heilongjiang, mainland China's easternmost county.

The agreement was met with controversy on both sides of the border. In May 2005, Cossacks in Khabarovsk demonstrated against the loss of half of Bolshoy Ussuriysky. In return, some Chinese media commentators in Hong Kong, Taiwan and overseas that are outside the control of the mainland Chinese government criticized the PRC for signing the agreement, which they regarded as sealing as permanent the loss of former Chinese territory such as Russian Manchuria to the RF.

The Government of the Republic of China (ROC) in Taipei has never recognized the sovereignty of the PRC government in Beijing. Therefore, the ROC still formally claims all parts of the Heixiazi Islands, as it does not consider border treaties signed by the PRC with other countries to be valid.

According to a 2002 study by Iwashita Akihiro, a Japanese specialist on Slavic relations, “Most of Khabarovsk’s local elites, in particular military, considered the islands of strategic importance since they fenced off Khabarovsk from China. If the border was drawn, relying upon the ‘main channel principle’, the two islands would have passed to China. This is why the Soviet Union insisted on the legal exceptionality of the two islands in its negotiations with China during the late 1980s, while strengthening its de facto control of these islands".

In 2023, the Chinese Global Times published the "2023 edition of China's standard map" that appeared to include all of the island's territory as China's.

The total area of these territories in the Khabarovsk region is approximately 340 square kilometres.

The Mainland Chinese section of the island is part of Fuyuan, Heilongjiang. The Russian section is part of Khabarovsky District of Khabarovsk Krai.

In 2015, the PRC registered the island as a nature reserve to protect biodiversity. The island hosts 505 species of flora and 351 species of fauna, 44 of which are nationally protected species, including the Siberian tiger. There is a "bear park" to contain black bears on the island.

• Sino-Russian border conflicts
• Treaty of Nerchinsk
• 1991 Sino-Soviet Border Agreement
• List of divided islands
• Abagaitu Islet

• Google Maps satellite image of Heixiazi/Bolshoy Ussuriysky and Yinlong islands
• 黑瞎子島紅太陽升起處 一個島兩個國 中國進行式 20170514 (in Chinese)

48°21′41″N 134°48′50″E / 48.36139°N 134.81389°E / 48.36139; 134.81389