Tianjin (; Chinese: 天津; pinyin: Tiānjīn; Mandarin: [tʰjɛ́n.tɕín] (listen)), alternately romanized as Tientsin (), is a municipality and a coastal metropolis in Northern China on the shore of the Bohai Sea. It is one of the nine national central cities in Mainland China, with a total population of 13,866,009 inhabitants during the 2020 Chinese census. Its built-up (or metro) area, made up of 12 central districts (all but Baodi, Jizhou, Jinghai and Ninghe), was home to 11,165,706 inhabitants and is also the world's 29th-largest agglomeration (between Chengdu and Rio de Janeiro) and 11th-most populous city proper.

It is governed as one of the four municipalities under the direct administration of Chinese central government and is thus under direct administration of the State Council. Tianjin borders Hebei Province and Beijing Municipality, bounded to the east by the Bohai Gulf portion of the Yellow Sea. Part of the Bohai Economic Rim, it is the largest coastal city in Northern China and part of the Jing-Jin-Ji megapolis.

In terms of urban population, Tianjin is the seventh largest city in China. In terms of administrative area population, Tianjin ranks fifth in Mainland China. The walled city of Tianjin was built in 1404. As a treaty port since 1860, Tianjin has been a major seaport and gateway to Beijing. During the Boxer Rebellion the city was the seat of the Tianjin Provisional Government. Under the Qing dynasty and the Republic of China, Tianjin became one of the largest cities in the region. At that time, numerous European-style buildings and mansions were constructed in concessions, many of which are well-preserved today. After the founding of the People's Republic of China, Tianjin suffered a depression due to the policy of the central government and 1976 Tangshan earthquake, but recovered from 1990s. Tianjin is classified as the largest type of port-city, a Large-Port Megacity, due to its large urban population and port traffic volume.

Nowadays, Tianjin is a dual-core city, with its main urban area (including the old city) located along the Hai River, which connects to the Yellow and Yangtze Rivers via the Grand Canal; and Binhai, an adjacent New Area urban core located east of the old city, on the coast of the Bohai Gulf. As of the end of 2010, around 285 Fortune 500 companies have set up base in Binhai. Since 2010, Tianjin's Yujiapu Financial District has become known as China's Manhattan and the city is considered one of the world's top 100 cities, including in the Global Financial Centres Index. Tianjin is ranked as a Beta (global second tier) city by the Globalization and World Cities Research Network.

Tianjin is one of the top 20 cities in the world by scientific research outputs as tracked by the Nature Index, and home to multiple notable institutes of higher education in Northern China, including Tianjin University, Nankai University, Tianjin Normal University, Tianjin Medical University, Tiangong University, Tianjin University of Science & Technology, Tianjin University of Technology, and Hebei University of Technology.