The Keelung campaign (August 1884–April 1885) was a military campaign undertaken by French forces in northern Formosa (Taiwan) during the Sino-French War. After making a botched attack on Keelung in August 1884, the French landed an expeditionary corps of 2,000 men and captured the port in October 1884. Unable to advance beyond their bridgehead, they were invested inside Keelung by superior Chinese forces under the command of the imperial commissioner Liu Mingchuan. In November and December 1884 cholera and typhoid drained the strength of the French expeditionary corps, while reinforcements for the Chinese army flowed into Formosa via the Pescadores Islands, raising its strength to 35,000 men by the end of the war. Reinforced in January 1885 to a strength of 4,500 men, the French won two tactical victories against the besieging Chinese in late January and early March 1885, but were not strong enough to exploit these victories. The Keelung campaign ended in April 1885 in a strategic and tactical stalemate. The campaign was criticised at the time by Admiral Amédée Courbet, the commander of the French Far East Squadron, as strategically irrelevant and a wasteful diversion of the French navy.

Following the defeat of China's Guangxi Army by the Tonkin Expeditionary Corps in the Bắc Ninh campaign (March 1884), a two-year confrontation between France and China in northern Vietnam was ended on 11 May 1884 by the conclusion of the Tientsin Accord, under which the Chinese undertook to withdraw their troops from Vietnam and to recognise a French protectorate in Tonkin. However, the hopes aroused by this agreement, which seemed to have brought France's Tonkin campaign to a victorious end, were shattered on 23 June 1884 by the Bắc Lệ ambush, in which a French column advancing to occupy Lạng Sơn and other frontier towns was attacked near Bắc Lệ by a detachment of the Guangxi Army.

When news of the 'Bắc Lệ Ambush' reached Paris, there was fury at what was perceived as blatant Chinese treachery. Jules Ferry's government demanded an apology, an indemnity, and the immediate implementation of the terms of the Tientsin Accord. The Chinese government agreed to negotiate, but refused to apologise or pay an indemnity. The mood in France was against compromise, and although negotiations continued throughout July, Admiral Amédée Courbet, the commander of France's newly created Far East Squadron, was ordered to take his ships to Fuzhou (Foochow). He was instructed to prepare to attack the Chinese fleet in the harbour and to destroy the Foochow Navy Yard. Meanwhile, as a demonstration of what would follow if the Chinese remained recalcitrant, Courbet was ordered on 2 August to despatch a naval force to the port of Keelung in northern Formosa, destroy its coastal defences, and occupy the town as a 'pledge' (gage) to be bargained against a Chinese withdrawal from Tonkin.

French interest in Keelung dated from February 1884, when Admiral Sébastien Lespès, the commander of France's Far East naval division, had recommended its occupation in the event of a war between France and China on the grounds that it could be easily seized and easily held. When this recommendation was made, Keelung was poorly defended. As tension between France and China grew in July 1884 the garrison of Keelung was substantially increased and the town's defences improved, and in August 1884 it was no longer the easy target it had been six months earlier. Its inner harbour was defended by three shore batteries: the recently completed Ta-sha-wan Battery (大沙灣砲台), boasted by the Chinese to be impregnable; the Ehr-sha-wan battery on the eastern side of the harbour; and a third battery on the seaward slope of Mount Clement (Huo-hao-shan, 火號山), a prominent hill to the west of the town. By the end of July 1884 there were 5,000 Chinese soldiers stationed in northern Formosa, deployed around the ports of Keelung and Tamsui. The appointment of Liu Mingchuan as imperial commissioner for Formosa in July 1884 also underscored the determination of the Qing government to defend Formosa. Liu was a veteran military commander, who had distinguished himself in the Taiping Rebellion.

Liu Mingchuan was aware that Keelung and the nearby port of Tamsui were likely targets for a French attack, and made sensible defensive dispositions to oppose a landing at both towns. He stationed 2,500 Chinese troops around Keelung, the likeliest target, under the command of the generals Sun Kaihua and Su Desheng, while he himself remained with the other half of his army at Tai-pak-fu (modern Taipei), occupying a central position that would allow him to move quickly either to Tamsui or to Keelung once the French threat developed.

Admiral Courbet, then aboard the French cruiser Volta in the Min River, received the French government's order to attack Keelung on the evening of 2 August. He entrusted this mission to Admiral Lespès, his second-in-command, who left the Min River the following morning aboard the gunboat Lutin to meet the ironclads Bayard and La Galissonnière off the island of Matsu. At Matsu, Lespès transferred Bayard's landing company to his flagship La Galissonnière, and on the night of 3 August crossed the Formosa Strait with La Galissonnière and Lutin. The two French ships arrived off Keelung on the morning of 4 August, where the cruiser Villars was already waiting for them.

On the morning of 5 August, after the Chinese rejected a French ultimatum to hand over their coastal defences, La Galissonnière, Villars and Lutin engaged and disabled Keelung's three coastal batteries. Admiral Lespès put a landing force ashore in the afternoon to occupy Keelung and the nearby coal mines at Pei-tao (Pa-tou, 八斗), but the arrival of a large body of Chinese troops led by Liu Mingchuan forced the French to make a fighting withdrawal and re-embark on 6 August. French casualties in this unsuccessful operation were 2 dead and 11 wounded. The Chinese suffered markedly heavier casualties.

The Chinese forts around Keelung contained 5 178mm guns in Fort Ehr-sha-wan and 3 48lb guns in Fort Ta-sha-wan with an unspecified amount of guns in Fort Huahaoshan.