The Rattanakosin Kingdom, the Kingdom of Siam, or the Bangkok Empire, were names used to reference the fourth and current Thai kingdom in the history of Thailand. It was founded in 1782 with the establishment of Rattanakosin (Bangkok), which replaced the city of Thonburi as the capital of Siam. This article covers the period until the Siamese revolution of 1932.

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.