Ancient Siam (also known as Ancient City, Thai: เมืองโบราณ, Mueang Boran) is a museum park constructed by Lek Viriyaphant and occupying over 200 acres (0.81 km²) in the shape of Thailand.

Ancient Siam is dubbed as the world's largest outdoor museum. Close to the Crocodile Farm in Samut Prakan Province, the 320-hectare "city" features 116 structures of Thailand's famous monuments and architectural attractions. The grounds of Ancient Siam correspond roughly to the shape of the kingdom, with the monuments lying at their correct places geographically. Some of the buildings are life-size replicas of existing or former sites, while others are scaled down. Still others are "creative designs," and not replicas of any historical structure.

The replicas were constructed with the assistance of experts from the National Museum to ensure historical accuracy. Outstanding works include the former Grand Palace of Ayutthaya (destroyed in the Burmese invasion of 1767), Phimai Sanctuary in Nakhon Ratchasima, and Wat Khao Phra Viharn on the Cambodian border.