In the mid-fourteenth century, the Siamese or Thai-speaking people from the Chao Phraya River plain rose to power.

After years of internal strife and in-fighting between the Khmers for the Angkorian throne, Angkor was slowly declining.

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.

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.

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.