The Treaty of Aigun (Russian: Айгунский договор; traditional Chinese: 璦琿條約; simplified Chinese: 瑷珲条约; pinyin: Àihún Tiáoyuē) was an 1858 treaty between the Russian Empire and Qing dynasty of China that established much of the modern border between the Russian Far East and Manchuria (the ancestral homeland of the Manchu people), which is now known as Northeast China. It reversed the Treaty of Nerchinsk (1689) by transferring the land between the Stanovoy Range and the Amur River from Qing China to the Russian Empire. Russia received over 600,000 square kilometres (231,660 sq mi) from Manchuria.
Since the reign of Catherine the Great (1762 – 1796), Russia had desired to become a naval power in the Pacific. It did so by annexing Kamchatka Peninsula and establishing the naval outpost of Petropavlovsk-Kamchatsky in 1740, naval outposts in Russian America and near the Amur watershed, encouraging Russians to go there and settle, and slowly developing a strong military presence in the Amur region. China never governed that region effectively or conducted territorial surveys, and these Russian advances went unnoticed.
From 1850 to 1864, China was heavily involved in fighting the Taiping Rebellion, and Governor-General of the Far East Nikolay Muraviev camped tens of thousands of troops on the borders of Mongolia and Manchuria, preparing to make legal Russian de facto control over the Amur from past settlement. Muraviev seized the opportunity when it was clear that China was losing the Second Opium War, and threatened China with a war on a second front. The Qing Dynasty agreed to enter negotiations with Russia.
The Russian representative Nikolay Muravyov and the Qing representative Yishan, both military governors of the area signed the treaty on May 28, 1858, in the town of Aigun.
The resulting treaty established a border between the Russian and Chinese Empires along the Amur River. (Chinese and Manchu residents of the Sixty-Four Villages East of the River would be allowed to remain, under the jurisdiction of Manchu government.) The Amur, Sungari, and Ussuri rivers were to be open exclusively to both Chinese and Russian ships. The territory bounded on the west by the Ussuri, on the north by the Amur, and on the east and south by the Sea of Japan was to be jointly administered by Russia and China—a "condominium" arrangement similar to that which the British and Americans had agreed upon for the Oregon Territory in the Treaty of 1818. (Russia gained sole control of this land two years later.)
In China, after the rise of Chinese nationalism and anti-imperialism in the 1920s, the treaty has been labelled an unequal treaty.