Under the Treaty of Nanking, an "unequal treaty" that ended the First Opium War, the Chinese island from which Hong Kong would grow was ceded to Britain.

The Treaty of Nanking was the peace treaty which ended the First Opium War (1839–1842) between Great Britain and the Qing dynasty of China on 29 August 1842. It was the first of what the Chinese later termed the "unequal treaties".

The unequal treaties were a series of agreements made between Asian countries—most notably Qing China, Tokugawa Japan and Joseon Korea—and Western countries—most notably the United Kingdom, France, Germany, Austria-Hungary, Italy, the United States and Russia—during the 19th and early 20th centuries. They were often signed following a military defeat suffered by the Asian party, or amid military threats made by the Western party. The terms specified obligations to be borne almost exclusively by the Asian party and included provisions such as the cession of territory, payment of reparations, opening of treaty ports, relinquishment of the right to control tariffs and imports, and granting of extraterritoriality to foreign citizens.

The First Opium War, also known as the Anglo-Chinese War, was a series of military engagements fought between the British Empire and the Chinese Qing dynasty between 1839 and 1842. The immediate issue was the Chinese enforcement of their ban on the opium trade by seizing private opium stocks from mainly British merchants at Guangzhou and threatening to impose the death penalty for future offenders. Despite the opium ban, the British government supported the merchants' demand for compensation for seized goods, and insisted on the principles of free trade and equal diplomatic recognition with China. Opium was Britain's single most profitable commodity trade of the 19th century. After months of tensions between the two states, the Royal Navy launched an expedition in June 1840, which ultimately defeated the Chinese using technologically superior ships and weapons by August 1842. The British then imposed the Treaty of Nanking, which forced China to increase foreign trade, give compensation, and cede Hong Kong Island to the British. Consequently, the opium trade continued in China. Twentieth-century nationalists considered 1839 the start of a century of humiliation, and many historians consider it the beginning of modern Chinese history.