Bangkok, officially known in Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand. The city occupies 1,568.7 square kilometres (605.7 sq mi) in the Chao Phraya River delta in central Thailand and has an estimated population of 9.0 million people as of 2021, 13% of the country's population. Over 17.4 million people (25%) live within the surrounding Bangkok Metropolitan Region as of the 2021 estimate, making Bangkok a megacity and an extreme primate city, dwarfing Thailand's other urban centres in both size and importance to the national economy.

Bangkok traces its roots to a small trading post during the Ayutthaya Kingdom in the 15th century, which eventually grew and became the site of two capital cities, Thonburi in 1767 and Rattanakosin in 1782. Bangkok was at the heart of the modernization of Siam during the late 19th century, as the country faced pressures from the West. The city was at the centre of the country's political struggles throughout the 20th century, as Siam—later renamed Thailand—abolished absolute monarchy, adopted constitutional rule, and underwent numerous coups and several uprisings. The city, incorporated as a special administrative area under the Bangkok Metropolitan Administration in 1972, grew rapidly during the 1960s through the 1980s and now exerts a significant impact on Thailand's politics, economy, education, media, and modern society.

The Asian investment boom in the 1980s and 1990s led many multinational corporations to locate their regional headquarters in Bangkok. The city is now a regional force in finance, business, and pop culture. It is an international hub for transport and health care, and has emerged as a centre for the arts, fashion, and entertainment. The city is known for its street life and cultural landmarks, as well as its red-light districts. The Grand Palace and Buddhist temples, including Wat Arun and Wat Pho, stand in contrast with other tourist attractions such as the nightlife scenes of Khaosan Road and Patpong. Bangkok is among the world's top tourist destinations and has been named the world's most visited city consistently in several international rankings.

Bangkok's rapid growth, coupled with little urban planning, has resulted in a haphazard cityscape and inadequate infrastructure. Despite an extensive expressway network, an inadequate road network and substantial private car usage have led to chronic and crippling traffic congestion, which caused severe air pollution in the 1990s. The city has since turned to public transport in an attempt to solve the problem, operating 10 urban rail lines and building other public transit; however, congestion remains a prevalent issue.