Bangkok, officially Krung Thep Maha Nakhon and colloquially Krung Thep, is the capital and most populous city of Thailand. The city covers an area of 1,568.7 square kilometres (605.7 sq mi) within the Chao Phraya River delta in central Thailand and had an estimated population of 10 million as of 2024, accounting for approximately 13% of the national population. As of the 2021 estimate, over 17.4 million people (about 25% of Thailand's population) resided within the surrounding Bangkok Metropolitan Region, making Bangkok a megacity and an exceptional primate city that far surpasses all other Thai urban centres in both scale and economic significance.

Bangkok traces its origins to a small trading post established during the Ayutthaya Kingdom in the 15th century. It subsequently developed into the site of two successive capitals: Thonburi in 1767 and Rattanakosin in 1782. The city played a pivotal role in the modernization of Siam during the late 19th century, as the nation sought to adapt amid Western imperial pressures. Throughout the 20th century, Bangkok remained at the centre of Thailand's political turbulence, witnessing the abolition of the absolute monarchy, the adoption of a constitutional system, and numerous coups and uprisings. Incorporated as a special local administrative area under the Bangkok Metropolitan Administration in 1972, the city experienced rapid expansion from the 1960s through the 1980s and has since exerted considerable influence over Thailand's politics, economy, education, media, and contemporary culture.

The Asian investment boom of the 1980s and 1990s attracted numerous multinational corporations to establish their regional headquarters in Bangkok. Today, the city serves as a major regional hub for finance, commerce, and pop culture. It also functions as an international centre for transport and healthcare and has become a prominent locus for the arts, fashion, and entertainment. Bangkok is widely recognized for its vibrant street life and cultural landmarks, as well as its red-light districts. The Grand Palace and several renowned Buddhist temples, including Wat Phra Kaew, Wat Arun, and Wat Pho, stand in contrast to modern tourist attractions such as the nightlife areas of Khaosan Road and Patpong. Consistently ranked among the world’s leading tourist destinations, Bangkok has frequently been named the most visited city globally in various international surveys.

Bangkok's rapid expansion, however, has been accompanied by limited urban planning, resulting in a sprawling cityscape and strained infrastructure. Despite an extensive expressway network, an insufficient road system combined with high levels of private vehicle ownership has led to chronic and severe traffic congestion, which in the 1990s contributed significantly to air pollution. In response, authorities have increasingly emphasized public transport solutions, leading to the development of ten urban rail lines and several ongoing expansion projects. Nonetheless, traffic congestion remains a persistent challenge in the city's urban landscape.

The history of Bangkok dates at least back to the early 15th century, to when it was a village on the west bank of the Chao Phraya River, under the rule of Ayutthaya. Because of its strategic location near the mouth of the river, the town gradually increased in importance. Bangkok initially served as a customs outpost with forts on both sides of the river, and was the site of a siege in 1688, in which the French were expelled from Siam. After the fall of Ayutthaya to the Burmese in 1767, the newly crowned King Taksin established his capital at the town, which became the base of the Thonburi Kingdom. In 1782, King Phutthayotfa Chulalok (Rama I) succeeded Taksin, moved the capital to the eastern bank's Rattanakosin Island, thus founding the Rattanakosin Kingdom. The City Pillar was erected on 21 April 1782, which is regarded as the date of the foundation of Bangkok as the capital.

Bangkok's economy gradually expanded through international trade, first with China, then with Western merchants returning in the early to mid-19th century. As the capital, Bangkok was the centre of Siam's modernization as it faced pressure from Western powers in the late 19th century. The reigns of Kings Mongkut (Rama IV, r. 1851–68) and Chulalongkorn (Rama V, r. 1868–1910) saw the introduction of the steam engine, printing press, rail transport and utilities infrastructure in the city, as well as formal education and healthcare. Bangkok became the centre stage for power struggles between the military and political elite as the country abolished absolute monarchy in 1932.

As Thailand allied with Japan in World War II, Bangkok was subjected to Allied bombing, but rapidly grew in the post-war period as a result of US aid and government-sponsored investment. Bangkok's role as a US military R&R destination boosted its tourism industry, as well as firmly establishing it as a sex tourism destination. Disproportionate urban development led to increasing income inequalities and migration from rural areas into Bangkok; its population surged from 1.8 million to 3 million in the 1960s.

Following the US withdrawal from Vietnam in 1973, Japanese businesses took over as leaders in investment, and the expansion of export-oriented manufacturing led to the growth of the financial market in Bangkok. Rapid growth of the city continued through the 1980s and early 1990s, until it was stalled by the 1997 Asian financial crisis. By then, many public and social issues had emerged, among them the strain on infrastructure reflected in the city's notorious traffic jams. Bangkok's role as the nation's political stage continues to be seen in strings of popular protests, from the student uprisings in 1973 and 1976, anti-military demonstrations in 1992, and frequent street protests since 2006, including those by groups opposing and supporting former prime minister Thaksin Shinawatra from 2006 to 2013, and a renewed student-led movement in 2020.