The Battle of San Marino was an engagement on 17–20 September 1944 during the Italian Campaign of the Second World War, in which German Army forces occupied the neutral Republic of San Marino and were then attacked by Allied forces. It is also sometimes known as the Battle of Monte Pulito.

San Marino had declared its neutrality earlier in the war and had remained broadly unaffected by events in Europe until 1943, when Allied forces had advanced a sizable distance up the Italian Peninsula. A major German defensive position, the Gothic Line, ran across the peninsula a short distance south of the Sammarinese border, and in late June 1944, the country was bombed by the Royal Air Force, killing 63 people, in the belief that the German Army had taken up positions on its territory. In Operation Olive, launched in late August, a strong Allied force attacked at the very eastern end of the line, aiming to pass through Rimini—just east of San Marino—and break out onto the plains north of the city. Whilst San Marino was southwest of Rimini, the plan was for it to be bypassed entirely. In response to the Allied movements, the Germans sent a small force into San Marino to guard their lines of communication and act as artillery observers.

After a few days, the main thrust of the offensive was halted south of Rimini by strong resistance and severe weather, and the British and Indian flanking forces began to push westwards, taking the frontline towards San Marino. On 17 September the 4th Indian Infantry Division attacked forces of the 278. Infanterie-Division holding two hills just across the Sammarinese border; after heavy fighting to gain control of the hills, the situation stabilised on the 19th, and Allied forces began to push into the City of San Marino itself. The city was captured by the afternoon of 20 September, and the 4th Indian Division left the country on the 21st, leaving it under the control of the local defence forces.

The microstate of San Marino, in the northern Italian Peninsula and fully surrounded by Italy, had played little role throughout the Second World War. It had a fascist government, closely aligned with Benito Mussolini's regime, but remained neutral. It was reported to have declared war against the United Kingdom in September 1940, though the Sammarinese government later transmitted a message to the British government stating that it had not. In early 1942, the Sammarinese government reiterated it was not at war with the United States, a position that was confirmed by the US State Department. The British Foreign Office noted more equivocally in 1944 that Britain had never declared war, but also had never formally recognised San Marino's neutrality, and that it felt that military action on Sammarinese territory would be justified if it were being used by Axis forces.

The country was bombed by the Allies on 27 June 1944, killing 63. The Sammarinese government declared the same day that no military installations or equipment were located on its territory, and that no belligerent forces had been allowed to enter. In early July, the Sammarinese government announced that prominent signs had been put up at the border crossings by the German command, to instruct German units not to enter the territory, and again reiterated its complete neutrality.

German troops were ordered in mid-August to respect the neutral status of San Marino, reserving the right for supply and medical services to transit the country, but in early September the German command acknowledged that it might deploy troops there during future operations, arguing "we cannot lose any battles for the sake of this little state". This left both Germany and the Allies acknowledging Sammarinese neutrality, but nonetheless prepared to move troops into the country if they felt it was required by operational needs.

By the late summer of 1944, German forces in Italy had withdrawn toward the Gothic Line, a chain of defended positions stretching across the Italian peninsula. The Allies formulated a plan to break through the defences, pushing north toward Rimini and the plains of Northern Italy. This would involve a strong thrust up the eastern seaboard by the British Eighth Army, codenamed Operation Olive; 11 divisions would attack along a narrow front, converging on the "Rimini Gap", an 8 mi (13 km) stretch of plain along the coast around the city, and then moving northward. Once through the Gap, the force would deploy outward onto the Romagna Plain, and move westward toward Bologna. Meanwhile, the American Fifth Army would push north along the centre of the peninsula, hopefully converging on Bologna and trapping a large German force in a pincer movement.

The main Allied assault began on 25 August, reaching the Foglia valley—the Gothic Line proper—on 29 August. It was quickly breached, and the German command attempted to assemble a second defensive line on the Coriano ridge, a hilly spur to the north of the Conca river, and the last major geographic obstacle south of Rimini. The Allied offensive reached the river on 3 September, but ground to a halt due to mechanical difficulties with its tanks, strengthening German resistance, and heavy rain. The Allied forces halted, and brought up reinforcements whilst waiting for a chance to resume the offensive along the coast. On the left flank of the assault, the attack had been halted in the Battle of Gemmano, to the south of the Conca river.