Pailin (Khmer: ប៉ៃលិន, Pailĭn [ˈpajlɨn]) is a province in western Cambodia at the northern edge of the Cardamom Mountains 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 and serving from 1994 to 1998 as the capital of the Provisional Government of National Union and National Salvation of Cambodia. Within Cambodia, Pailin is known for its natural resources, namely precious gems and timber.

Once a part of the powerful Khmer Empire, Pailin was conquered in 1558 by the Burmese under Bayinnaung and later ruled by the Siamese (Thai) until 1907 and from 1941 to 1946, when it was returned to Cambodia. It was known to the Thai as "Phailin" (Thai: ไพลิน, Thai pronunciation: [pʰāj.lin]). There is still a vibrant border crossing point in Pailin. On 22 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 City (Krong Pailin) was founded under the Khom (ขอม)Empire, where it existed as an important trading centre. It continued its role as a centre of trade and commerce under French colonial rule, which saw the establishment of French-operated stores and plantations. During the late 1800s, the area saw an influx of immigrants from Burma who became the Kula people. The Kula settled around Pailin and established structures whose architecture was based on the architecture of Burma.

In the early 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 Khmer Republic 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 prince Norodom Sihanouk had allied himself with the Khmer Rouge, and most residents believed that they were fighting to restore him to power. It was not long 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 Khmer Rouge used proceeds from mining in the Pailin area to fund their offensive and later their government once they gained national power. When the invading People's Army of Vietnam ousted them from power, they retreated to Pailin, where many former Khmer Rouge leaders remain today. It was the capital of the Provisional Government of National Union and National Salvation of Cambodia.

Some leaders went into hiding in fear of punishment for their crimes, although other leaders lived openly in the province. 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.

Since the war, Pailin has suffered an economic decline and failures of local businesses. However, the now politically stable area is seeing a new wave of tourism focused on its ancient temples, natural forests, and wildlife, and especially the precious gem market.

Until 2001, Pailin was part of Battambang Province, then elevated to city/municipal status, and then again to provincial status, and thus became an autonomous zone of its own.

The current governor of Pailin province is Ban Sreymom, appointed by royal decree on 18 December 2020. Sreymom is a member of the ruling Cambodian People's Party and previously represented Pailin constituency in the National Assembly. She is the second woman in Cambodian history to serve as a provincial governor.