Nanjing is the capital of Jiangsu, a province in East China. The city, which is located in the southwestern corner of the province, has 11 districts, an administrative area of 6,600 km² (2,500 sq mi), and as of 2021^[update] a population of 9,423,400. Situated in the Yangtze River Delta, Nanjing has a prominent place in Chinese history and culture, having served as the capital of various Chinese dynasties, kingdoms and republican governments dating from the 3rd century to 1949, and has thus long been a major center of culture, education, research, politics, economy, transport networks and tourism, being the home to one of the world's largest inland ports. The city is also one of the fifteen sub-provincial cities in the People's Republic of China's administrative structure, enjoying jurisdictional and economic autonomy only slightly less than that of a province. It has also been awarded the title of 2008 Habitat Scroll of Honor of China, Special UN Habitat Scroll of Honor Award and National Civilized City. Nanjing is also considered a Beta (global second-tier) city classification, together with Chongqing, Hangzhou and Tianjin by the Globalization and World Cities Research Network, and ranked as one of the world's top 100 cities in the Global Financial Centres Index.

As of 2021, Nanjing has 68 institutions of higher learning, including 13 double-first-class universities, ten 111-plan universities, eight 211 universities, and 97 academies. Nanjing University, which has a long history, is among the world's top 10 universities ranked by the Nature Index. The ratio of college students to the total population ranks No.1 among large cities nationwide. Nanjing has the fifth-largest scientific research output of any city in the world. As of 2024, it has been ranked as the world's second most prolific scientific research center in earth and environmental sciences and the world's third most prolific scientific research center in chemistry and physical sciences, according to the Nature Index.

Nanjing, one of the nation's most important cities for over a thousand years, is recognized as one of the Four Great Ancient Capitals of China. It has been one of the world's largest cities, enjoying peace and prosperity despite various wars and disasters. Nanjing served as the capital of Eastern Wu (229–280), one of the three major states in the Three Kingdoms period; the Eastern Jin and each of the Southern dynasties (Liu Song, Southern Qi, Liang and Chen), which successively ruled southern China from 317 to 589; the Southern Tang (937–75), one of the Ten Kingdoms; the Ming dynasty when, for the first time, all of China was ruled from the city (1368–1421); and the Republic of China under the nationalist Kuomintang (1927–37, 1946–49) before its flight to Taiwan by Chiang Kai-Shek during the Chinese Civil War. The city also served as the seat of the rebel Taiping Heavenly Kingdom (1853–64) and the Japanese puppet regime of Wang Jingwei (1940–45) during the Second Sino-Japanese War. It suffered many notable devastating atrocities in both conflicts, most notably the Nanjing Massacre from late 1937 to early 1938.

Nanjing became the capital city of Jiangsu province in 1952, after serving as a Direct-administered Municipality from 1949 to 1952 following the establishment of the People's Republic of China. It has many important heritage sites, including the Presidential Palace, Sun Yat-sen Mausoleum and Ming Xiaoling Mausoleum. Nanjing is famous for human historical landscapes, mountains and waters such as Fuzimiao, Ming Palace, Chaotian Palace, Porcelain Tower, Drum Tower, Stone City, City Wall, Qinhuai River, Xuanwu Lake and Purple Mountain. Key cultural facilities include Nanjing Library, Nanjing Museum and Jiangsu Art Museum.

The name Nanjing ("Southern Capital") was originally informal, appearing in Xiao Zixian's 6th-century reply to Xiao Tong during the Northern and Southern Dynasties era of Chinese history. Synonyms were also used, like Nandu (南都, Nándū). Under the Hongwu Emperor who founded the Ming dynasty, after the abandonment of plans for a third capital at Fengyang, a distinction began to be made between his northern capital at Kaifeng and the southern one at Yingtian (應天, Yìngtiān, "Following Heaven"). This distinction was continued and eventually formalized after his son the Yongle Emperor relocated his court to Shuntian or Beijing ("Northern Capital"). The continuation of the dual arrangement was required to respect the wishes of his father, whose Ancestral Injunctions had insisted Nanjing should remain a permanent imperial capital. The Nanjing form of Lower Yangtze Mandarin remained a prestige dialect and the imperial lingua franca for centuries, producing formerly common romanizations of the name as Nanqim, Nankin, and Nanking. The less common Wade-Giles form of Nan-ching was an earlier attempt to represent its pronunciation in the Beijing form of Mandarin, now represented in pinyin as Nánjīng.

The city has a number of other names, and some historical names are now used as names of districts of the city.

During the Warring States Era, settlements within modern Nanjing were known as Yuecheng (越城, Yuèchéng, "Yue City") and Jinlingyi (金陵邑, Jīnlíngyì, "City of the Golden" or "Precious Burial Mound") or Jinling (金陵, Jīnlíng), from which Nanjing is sometimes known^{[citation needed]} as Jincheng (金城, Jīnchéng, "Golden City"). Under the Qin, Jinling was renamed Moling (秣陵, Mòlíng, "Fodder Mound").

Jianye (建業, Jiànyè, "Establishing Merit") was adopted as the name of the Wu capital during the Three Kingdoms Era. The city first became an imperial Chinese capital under the Sima Jin dynasty under the name Jiankang, a change adopted to avoid the naming taboo occasioned by the elevation of Emperor Min, whose personal name was Sima Ye. Under the Tang dynasty, it was known as Shengzhou (昇州, Shēngzhōu, "Ascending Prefecture").