The Tank, Infantry, Mk III, Valentine was an infantry tank produced in the United Kingdom during World War II. More than 8,000 Valentines were produced in eleven marks, plus specialised variants, accounting for about a quarter of wartime British tank production. The variants included riveted and welded construction, petrol and diesel engines and increases in armament. It was supplied in large numbers to the USSR and built under licence in Canada. It was used by the British in the North African campaign. Developed by Vickers, it proved to be strong and reliable.

There are several proposed explanations for the name Valentine. According to the most popular one, the design was presented to the War Office on St Valentine's Day, 14 February 1940, although some sources say that the design was submitted on Valentine's Day 1938 or 10 February 1938. White notes that "incidentally" Valentine was the middle name of Sir John Carden, the man responsible for many tank designs including that of the Valentine's predecessors, the A10 and A11. Another version says that Valentine is an acronym for Vickers-Armstrongs Limited Elswick & (Newcastle-upon) Tyne. The "most prosaic" explanation according to author David Fletcher is that it was just an in-house codeword of Vickers with no other significance.

The Valentine started as a proposal based on Vickers' experience with the A9 and A10 specification cruiser tanks and the A11 (Infantry Tank Mk I). As a private design by Vickers-Armstrongs, it did not receive a General Staff "A" designation; it was submitted to the War Office on 10 February 1938. The development team tried to match the lower weight of a cruiser tank, allowing the suspension and transmission parts of the A10 heavy cruiser to be used, with the greater armour of an infantry tank, working to a specification for a 60 mm (2.4 in) armour basis (the same as the A.11).

The tank was to carry a 2-pounder gun in a two-man turret (the A.11 was armed only with a heavy machine gun), a lower silhouette and be as light as possible, resulting in a very compact vehicle with a cramped interior. Compared to the earlier Infantry Tank Mk II "Matilda", the Valentine had somewhat weaker armour and almost the same top speed. By using components already proven on the A9 and A10, the new design was easier to produce and much less expensive.

The War Office was initially deterred by the size of the turret, since they considered a turret crew of three necessary, to free the vehicle commander from direct involvement in operating the gun. Concerned by the situation in Europe, it finally approved the design in April 1939 and placed the first order in July for deliveries in May 1940. At the start of the war, Vickers were instructed to give priority to the production of tanks. The vehicle reached trials in May 1940, which coincided with the loss of much of the army's equipment in France, during Operation Dynamo, the evacuation from Dunkirk. The trials were successful and the vehicle was rushed into production as "Tank, Infantry, Mark III"; no pilot models were required as much of the mechanics had been proven on the A10, and 109 had been built by the end of September. During late 1940 and early 1941, Valentines were used in the cruiser tank role in British-based armoured divisions, and they were supplied to tank brigades of the Eighth Army in North Africa from June 1941.

Metropolitan-Cammell Carriage & Wagon—an associate company of Vickers—and Birmingham Railway Carriage & Wagon Company (BRCW) were contracted to produce the Valentine. Metropolitan and the BRCW had built small numbers of the A10, their production runs were just finishing and they delivered their first Valentines in mid-1940. Metropolitan used two sites, with Wednesbury joined by their Midland site in production of the Valentine. Vickers output started at ten per month rising to 45 per month in a year and peaking at 20 per week in 1943, before production was slowed and then production of the Valentine and derivatives stopped in 1945. Vickers-Armstrong produced 2,515 vehicles and Metropolitan 2,135; total UK production was 6,855 tanks, with 2,394 exported from Britain to the Soviet Union under lend-lease.

To develop its own tank forces, Canada had established tank production facilities. An order was placed in 1940 with Canadian Pacific and after modifications to the Valentine design to use local standards and materials, the production prototype was finished in 1941. Canadian production was mainly at CPR Angus Shops in Montreal and 1,420 were produced in Canada, of which 1,388 were sent to the Soviet-Union. They formed the main Commonwealth export to the Soviet Union under lend-lease. The remaining 32 were retained for training. The use of local GMC Detroit Diesel two-stroke engines in Canadian production was a success and the engine was adopted for British production. British and Canadian production totalled 8,275, making the Valentine the most produced British tank design of the war.

The Valentine was of conventional layout, divided internally into three compartments; from front to back the driver's position, the fighting compartment with the turret and then the engine and transmission driving the tracks through rear sprockets. The driver's area contained only the driver and the driving controls. The driver sat on hull centre line, entering through either of two angled hatches over the seat, though there was an emergency exit hatch beneath his seat. The driver had a direct vision port—cut in what was one of the hull cross members—in front of him and two periscopes in the roof over his head. Driving was by clutch and brake steering through levers, whose control rods ran the length of the hull to the transmission at the rear.