| Native Name: | Siam Nakhon / Siemmarat |
| Common Name: | Siam Nakhon Province |
| Subdivision: | Province |
| Nation: | Siam |
| Year Start: | 1867 |
| Year End: | 1907 |
| Event End: | Ceded to France |
| S1: | French Protectorate of Cambodia |
| Flag S1: | Flag of Cambodia under French protection.svg |
| Flag Type: | Flag |
| Image Map Caption: | Map of Siam Nakhon Province in 1900 |
| Today: | Cambodia |
Siam Nakhon (Thai: สยามนคร), later known as Siam Marat, was the name of a former Thai province in the Angkor area. It was a changwat (province) of Siam, but was ceded to French Indochina following the Franco–Siam Treaty of 1906. Its Khmer name was Siem Reap, which meant "defeat Siam".[1]
Angkor, the capital city of the Khmer Empire in ancient times, was declining in the 17th century. In the mid-fourteenth century, the Siamese or Thai-speaking people from the Chao Phraya River plain rose to power. The new Siamese kingdom of Ayutthaya was founded and started expanding eastwards, towards Angkor.
After years of internal strife and in-fighting between the Khmers for the Angkorian throne, Angkor was slowly declining. Eventually, Angkor Thom was sacked and abandoned until the nineteenth century.
Prior to the Franco-Thai Treaty of 1867, the provinces of Siem Reap and Battambang were ceded to Siam in 1795 by the Cambodian king, Ang Eng, who in return was able to rule Cambodia without interference from Aphaiphubet, a Thai-backed Khmer officer. The treaty ensured that the two provinces were part of Siam proper in return for Siam renouncing suzerainty over the rest of Cambodia. The city of Siem Reap was under total Siamese control through a local Khmer family. The Siamese then called this province Siam Nakhon, (later known as Siemmarat), meaning "Siamese town."
At the beginning of the 20th century, the province (along with Battambang) was in turn ceded back to Cambodia (now a part of French Indochina) in the Franco-Siamese Treaty of 1907, which replaced the 1904 version of the treaty in which the French had accidentally annexed the Thai-speaking city of Trat. Because of the historical relationships of the court of Angkor to Vietnam (also a part of French Indochina), the history of Angkor as the capital of the Khmer empire and France's desire for the complete territorial integrity of French Indochina, the French were able to exchange the provinces of Siem Reap and Battambang with Siam while Siam gained both Trat and the district of Dan Sai.
The province reverted to its previous name of Siem Reap, which in Khmer means "defeat of Siam", referring to the defeat that the Siamese suffered during their invasion of Angkor in the year 1540, after which King Chan Reachea (r. 1516-1566) had renamed the province from Mahanokor to Siem Reap.