The city of Rattanakosin, now known internationally as Bangkok, is founded on the eastern bank of the Chao Phraya River by King Buddha Yodfa Chulaloke.

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 10 million people as of 2024, 13% of the country's population. Over 17.4 million people 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.

The history of Bangkok, the capital of Thailand, dates at least to the early 15th century, when it was under the rule of Ayutthaya. Due to its strategic location near the mouth of the Chao Phraya River, the town gradually increased in importance, and after the fall of Ayutthaya King Taksin established his new capital of Thonburi there, on the river's west bank. King Phutthayotfa Chulalok, who succeeded Taksin, moved the capital to the eastern bank in 1782, to which the city dates its foundation under its current Thai name, "Krung Thep Maha Nakhon". Bangkok has since undergone tremendous changes, growing rapidly, especially in the second half of the 20th century, to become the primate city of Thailand. It was the centre of Siam's modernization in the late 19th century, subjected to Allied bombing during the Second World War, and has long been the modern nation's central political stage, with numerous uprisings and coups d'état having taken place on its streets throughout the years.

Phutthayotfa Chulalok, also known by his regnal name Rama I, was the founder of the Rattanakosin Kingdom and the first King of Siam from the reigning Chakri dynasty. He ascended the throne in 1782, following the deposition of King Taksin of Thonburi. He was also celebrated as the founder of Rattanakosin as the new capital of the reunited kingdom.