The Marco Polo Bridge incident, also known as the Lugou Bridge incident or the July 7 incident, was a 3-day battle that began on 7 July 1937 in the district of Beijing between the 29th Army of the National Revolutionary Army of the Republic of China and the Imperial Japanese Army.

Since the Japanese invasion of Manchuria in 1931, there had been many small incidents along the rail line connecting Beijing with the port of Tianjin, but all had subsided. On the night of 7 July, Japanese garrison troops at Lugouqiao held an unusual manoeuvre; and, alleging that a Japanese soldier was missing, demanded entry into the City of Wanping to conduct a search. Fighting broke out while the Japanese complaint was still under negotiation. However, the missing Japanese soldier had already returned to his lines. The Marco Polo Bridge incident is generally regarded as the start of the Second Sino-Japanese War.

In English, the battle is usually known as the "Marco Polo Bridge incident". The Marco Polo Bridge is an eleven-arch granite bridge, an architecturally significant structure first erected under the Jin dynasty and later restored during the reign of the Kangxi Emperor of the Qing dynasty in 1698. It gained its Western name from its appearance in Il Milione, Marco Polo's record of his travels.

It is also known as the "Lukouchiao", "Lugouqiao", or 'Lugou Bridge incident' from the local name of the bridge, derived from a former name of the Yongding River. This is the common name for the event in Japanese (蘆溝橋事件, Rokōkyō Jiken) and is an alternate name for it in Chinese and Korean (Korean: 노구교 사건; RR: Nogugyo Sageon). The same name is also expressed or translated as the "Battle of Lugou Bridge", "Lugouqiao", or "Lukouchiao".

Tensions between the Empire of Japan and the Republic of China had been heightened since the Japanese invasion of Manchuria in 1931 and their 1932 creation of a puppet state, Manchukuo, with Puyi, the deposed Qing dynasty emperor, as its head of state. After the invasion, Japanese forces extended their control further into northern China, seeking to obtain raw materials and industrial capacity. A commission of inquiry from the League of Nations published the Lytton Report which was critical of the Japanese, resulting in Japan quitting the League.

The Kuomintang (KMT) government of China refused to recognize Manchukuo but did agree to the Tanggu Truce with Japan in 1933. Subsequently, there were various "incidents", or armed clashes of a limited nature, followed by a return to uneasy peace. The significance of the Marco Polo Bridge incident is that, following it, tensions did not subside again; instead, there was an escalation, with larger forces committed by both sides and fighting spreading to other parts of China. With hindsight, this small incident can, therefore, be regarded as the start of a major conflict.

By the terms of the Boxer Protocol of 7 September 1901, China had granted nations with legations in Beijing the right to station guards at twelve specific points along railways connecting Beijing with Tianjin. This was to ensure open communications between the capital and the port. By a supplementary agreement on 15 July 1902, these forces were allowed to conduct maneuvers without informing the authorities of other nations in China.

By July 1937, Japan had expanded its forces in China to an estimated 7,000 to 15,000 men, mostly along the railways. This number of men, and the amount of concomitant matériel, was several times the size of the detachments deployed by the European powers, and greatly in excess of the limits set by the Boxer Protocol. By this time, the Imperial Japanese Army had already surrounded Beijing and Tianjin.