The Tonkin Expeditionary Corps (French: corps expéditionnaire du Tonkin) was a French military command based in northern Vietnam (Tonkin) from June 1883 to April 1886. The expeditionary corps fought the Tonkin Campaign (1883–86) taking part in campaigns against the Black Flag Army and the Chinese Yunnan and Guangxi Armies during the Sino-French War (August 1884–April 1885) and the period of undeclared hostilities that preceded it (August 1883–June 1884), and in important operations against Vietnamese guerrilla bands during the subsequent 'Pacification of Tonkin' (May 1885–February 1886).

The expeditionary corps was established in June 1883 in the wake of Henri Rivière's defeat and death at the Battle of Paper Bridge, to entrench the French protectorate in Tonkin. Its first commander was général de brigade Alexandre-Eugène Bouët (1833–87), the most senior marine infantry officer available in the French colony of Cochinchina. Bouët introduced a lightweight black pyjama summer uniform for French troops in Tonkin, and also ordered them to cover their white pith helmets with black cloth to make themselves less conspicuous. These were sensible innovations, which were appreciated by the common soldiers. Bouët's first task was to secure the French posts in Hanoi, Nam Định and Haiphong against Black Flag and Vietnamese attacks. In July 1883 he prepared to go over to the offensive, recruiting a force of 800 Yellow Flag soldiers to augment the French forces at his disposal. Pressured by the civil commissioner-general Jules Harmand to attack the Black Flags as soon as possible in their positions on the Day River, he took the field in August 1883, despite the heat and humidity of the Tonkin summer. Bouët twice attacked the entrenchments held by Liu Yongfu's Black Flag army, at the Battle of Phủ Hoài (15 August 1883) and the Battle of Palan (1 September 1883). Both battles were disappointing for the French, and Bouët's command was also dogged by disagreements with Jules Harmand over French strategy in Tonkin. In early September 1883 Bouët resigned. Shortly before his resignation he recommended to the French government that the Tonkin Expeditionary Corps should be constituted as a regular two-brigade army division with the normal complement of artillery and ancillary support. This recommendation was accepted by the army ministry and implemented in February 1884.

In the wake of Bouët's unexpected resignation, command of the expeditionary corps fell briefly to Lieutenant-Colonel Anicet-Edmond-Justin Bichot (1835–1908), the ranking marine infantry officer in Tonkin. Bichot's interregnum lasted only a few weeks, and saw no large-scale battles between the French and the Black Flags. His short tenure of command was chiefly notable for a reconnaissance in force towards Sơn Tây on 18 September 1883, in the course of which the French discovered the remains of Commandant Henri Rivière, who had been killed in action at the Battle of Paper Bridge (19 May 1883) and whose mutilated body had been buried close to the battlefield, in the village of Kien Mai.

Upon the arrival of substantial French reinforcements in Tonkin in November and December 1883, the French government placed Admiral Amédée Courbet, the commander of the Tonkin Coasts naval division, in command of the expeditionary corps. Courbet commanded the expeditionary corps during the Sơn Tây Campaign (December 1883). Among his many other talents, Courbet was a first-class administrator, and was deeply respected by the men of the expeditionary corps for the pains he took to ensure that military life ran as smoothly as possible for them. Long after his departure, this aspect of his command was remembered. If the mail went astray, or if a unit lost its way during a route march, the soldiers would say 'This wouldn't have happened if Courbet were still in command!'

The 1883 campaigns in Tonkin were conducted, like most French colonial enterprises, by the troupes de marine, and had been overseen by the navy ministry. In December 1883, however, in view of the increasing commitment of troops from Algeria to Tonkin, the army ministry insisted on appointing a general from the regular army to command of the Tonkin Expeditionary Corps, which would be henceforth be constituted as a two-brigade infantry division with the normal complement of artillery and other supporting arms. Jules Ferry's cabinet approved this recommendation, and Courbet was replaced in command of the expeditionary corps on 16 December 1883 by General Charles-Théodore Millot—ironically, on the very day on which he captured Sơn Tây. He resumed command of the Tonkin Coasts naval division, and for the next six months played a most unwelcome subordinate role, hunting down bands of Vietnamese pirates in the Gulf of Tonkin while Millot was winning glory in the Bắc Ninh campaign.

In early 1884 the arrival of substantial reinforcements from France and the African colonies raised the strength of the expeditionary corps to over 10,000 men. Its new commander, général de division Charles-Théodore Millot (1829–1889), organised this force into two brigades. The 1st Brigade was commanded by général de brigade Louis Brière de l'Isle (1827–1896), who had earlier made his reputation as governor of Senegal. The 2nd Brigade was commanded by général de brigade François de Négrier (1842–1913), a charismatic young Foreign Legion commander who had recently quelled a serious Arab rebellion in Algeria.

Millot commanded the expeditionary corps for eight months, from February to September 1884. During his tenure of command he organised two major campaigns to capture Bắc Ninh and Hưng Hóa (March and April 1884) and two more modest expeditions to capture Thái Nguyên and Tuyên Quang (May and June 1884). In the Bắc Ninh campaign he won a spectacular walkover victory against Xu Yanxu's Guangxi Army. In the Hưng Hóa campaign he flanked Liu Yongfu out of a formidable defensive system without losing a man. Having argued strenuously against sending a column to occupy Lạng Sơn in the heat of the Tonkin summer, he emerged unscathed from the official enquiry into the circumstances of the Bắc Lệ Ambush (June 1884).

Millot was arguably the most successful of the many commanders of the expeditionary corps, but he was unpopular with both his officers and his men, who considered him overcautious. Significantly, his decision to halt General de Négrier's pursuit of the defeated Chinese forces in the Bắc Ninh campaign was held against him, even though he had sound military reasons for this decision. The troops immediately gave sardonic Vietnamese nicknames to their three generals. The much-admired de Négrier became Maolen ('Quick!'), Brière de l'Isle Mann Mann ('Slow!'), and Millot Toi Toi ('Stop!). Millot's career in Tonkin ended on a sour note. In poor health, and dismayed at the way the French government used the Bắc Lệ ambush as a pretext for war with China, he submitted his resignation in September 1884. In his last order of the day, he described himself as 'a sick and disappointed man'.