The Battle of Wuhan (traditional Chinese: 武漢會戰; simplified Chinese: 武汉会战; Japanese: 武漢作戦 (ぶかんさくせん)), popularly known to the Chinese as the Defence of Wuhan (traditional Chinese: 武漢保衛戰; simplified Chinese: 武汉保卫战), and to the Japanese as the Capture of Wuhan, was a large-scale battle of the Second Sino-Japanese War. Engagements took place across vast areas of Anhui, Henan, Jiangxi, Zhejiang, and Hubei provinces over a period of four and a half months. It was the single largest, longest, and bloodiest battle of the entire Second Sino-Japanese War. More than one million National Revolutionary Army troops from the Fifth and Ninth War Zone were put under the direct command of Chiang Kai-shek, defending Wuhan from the Central China Area Army of the Imperial Japanese Army led by Shunroku Hata. Chinese forces were also supported by the Soviet Volunteer Group, a group of volunteer pilots from Soviet Air Forces.

Although the battle ended with the eventual capture of Wuhan by the Japanese forces, it resulted in heavy casualties, with China suffering as many as one million casualties, military and civilian. With Japan suffering its heaviest losses of the war, it decided to divert its attention to the north, which would prolong the war until the attack on Pearl Harbor. The end of the battle signaled the beginning of a strategic stalemate in the war, shifting from large pitched battles to localised struggles.

On 7 July 1937, the Imperial Japanese Army (IJA) launched a full-scale invasion of China after the Marco Polo Bridge Incident. Both Beijing and Tianjin had fallen to the Japanese by 30 July, which exposed the rest of the North China Plain. To disrupt the Japanese invasion plans, the Chinese Nationalists decided to engage the Japanese in Shanghai, which opened a second front. The fighting lasted from 13 August to 12 November, with the Chinese suffering major casualties including "70 percent of Chiang Kai-shek's young officers." After the fall of Shanghai, Nanjing, which was the capital of China, was threatened directly by the Japanese forces. The Nationalists were thus forced to declare the capital an open city while they began the process of moving the capital to Chongqing.

With the fall of three major Chinese cities (Beijing, Tianjin, and Shanghai), there was a large number^[vague] of refugees fleeing the fighting in addition to the government facilities and war supplies that needed to be transferred to Chongqing. Inadequacies in the transport systems prevented the government from being able to complete the transfer. Wuhan thus became the "de facto wartime capital" of the Republic of China for its strong industrial, economic, and cultural foundations. Assistance from the Soviet Union provided additional military and technical resources, including the Soviet Volunteer Group.

On the Japanese side, the IJA forces were drained by the large number and the extent of military operations since the beginning of the invasion. Reinforcements were thus dispatched to boost forces in the area, which placed a strain on the Japanese peacetime economy, leading Prime Minister Fumimaro Konoe to reassemble his Cabinet in 1938 and to introduce the National Mobilization Law on 5 May that year, which moved Japan into a wartime economic state.

Wuhan, located halfway upstream of the Yangtze River, was the second-largest city in China, with a population of 1.5 million in late 1938. The Yangtze River and the Hanshui River divide the city into three regions, which include Wuchang, Hankou and Hanyang. Wuchang was the political center, Hankou was the commercial district, and Hanyang was the industrial area. After the completion of the Yuehan Railway, the importance of Wuhan as a major transportation hub in the interior of China was further established. The city also served as an important transit point for foreign aid moving inland from the southern ports.

After the Japanese capture of Nanjing, the bulk of the Nationalist government agencies and the military command headquarters were in Wuhan although the capital had been moved to Chongqing. Wuhan thus became the de facto wartime capital at the onset of the engagements in Wuhan. The Chinese war effort was thus focused on protecting Wuhan from being occupied by the Japanese. The Japanese government and the headquarters of the China Expeditionary Army expected Wuhan to fall, along with the Chinese resistance, "within a month or two."

In December 1937, the Military Affairs Commission was created to determine the battle plan for the defense of Wuhan. After the loss of Xuzhou, approximately 1.1 million men or 120 divisions of the National Revolutionary Army were redeployed. The commission decided to organize the defense around the Dabie Mountains, Poyang Lake, and the Yangtze River against the 200,000 Japanese in 20 divisions of the Imperial Japanese Army. Generals Li Zongren and Bai Chongxi of the Fifth War Zone were assigned to defend the north of the Yangtze, and Chen Cheng of the Ninth War Zone was tasked with defending the south. The First War Zone, located in the west of the Zhengzhou-Xinyang section of the Pinghan Railway, was given the task of stopping the Japanese forces coming from the North China Plain. Finally, Chinese troops in the Third War Zone, located between Wuhu, Anqing and Nanchang, were given the task to protect the Yuehan Railway.