The First Capture of Chusan by British forces in China occurred on 5–6 July 1840 during the First Opium War. The British captured Chusan (Zhoushan), the largest island of an archipelago of that name.

The First Opium War, also known as the Anglo-Chinese War, was a series of military engagements fought between the British Empire and the Qing dynasty of China between 1839 and 1842. The immediate issue was the Chinese enforcement of their ban on the opium trade by seizing private opium stocks from 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.