Pailin is a province at the northern edge of the Cardamom Mountains, in the west of Cambodia near the border of Thailand. This province is surrounded by Battambang Province, and was officially carved out of Battambang to become a separate administrative division after the surrender of the Ieng Sary faction of the Khmer Rouge in 1996. Pailin is known to much of the world for having long been a stronghold of the Khmer Rouge, remaining under their control long after they were defeated in 1979.

On 22nd December, 2008, King Norodom Sihamoni signed a Royal Decree that changed the municipalities of Kep, Pailin and Sihanoukville into provinces, as well as adjusting several provincial borders.

Pailin was originally a small forest zone full of animal and several precious stones. Pailin is the transform word from the original Pan Len , referred by a tale about around the 18th or 19th century, there were a group of hunters who chased the wild animals farther and farther until they reached the present-day Pailin where, then, was of jungles of high trees and some fields by the canal which was called small river such as Tong small river. The hunters saw otters that were swimming and they ran away when they saw people. Then, the hunters went to see the place where the otters played together. They saw small bright rocks and everybody took a rock to show their friends and their relatives in their home land. Arriving in the home land, there were residents (it was said they were Kula[disambiguation needed] people in Chanthaburi Province Siam) knew that the rocks were valuable so they took and broke the rocks to polish into gems with good purity. That was why they went to buy gems from the hunters. The hunters knew that it was way of earning a living, they often tried to go to take the rocks from the old place. At that time, Pailin began to faced as the business place.

The frequent going and coming from that place made them call Phee Leng  due to the playing of the otters that they saw at the first time. But the residents in Siam pronounced Phee as Phai and Leng as Len​​​​ (Khmer:ឡេន)​ or Lin with their Siamese sound as Pea Lean During the control of France of that area, it was written in Latin with the sound and morphology of the words as Phailin. Later the word was shortened as Pailin by omitting " H "​ាaim of a difficult pronunciation.

In the late 1970s, Pailin was a prosperous town stemming from the extensive gem deposits in the surrounding countryside. Because of its resources, it was one of the first cities invaded by the Khmer Rouge when they began their major offensive against the national government. The city offered no resistance and the Khmer Rouge soldiers were greeted as liberators as they marched into town. At this point, the deposed king had allied himself with the KR and most residents believed that the KR were fighting to restore him to power. It was not long, though, before most residents were rounded up for a forced march to the countryside to work in rice paddies. Those believed to have connections to the government were killed. The KR used proceeds from mining in the Pailin area to fund their offensive and later their government once they gained national power. When the invading Vietnamese Army threw them from power, they retreated to Pailin, where many former KR leaders remain today.

Some leaders went into hiding in fear of punishment for their crimes, although other leaders lived openly in the province. It is said that 70% of the area's older men were fighters for the Khmer Rouge,[who?] but none of the regular fighters have yet been brought to justice.[citation needed] As of September 2007, Pailin's remaining Khmer Rouge leaders were being rounded up to face justice by an international tribunal, including Khieu Samphan and Nuon Chea.

In the recent years, Pailin suddenly meet the decline of Economy and Business. However, a new wave of tourism began found depend on its ancient temple, natural forest, animals and especially the precious stones.