Kanchanaburi (กาญจนบุรี, pronounced [kāːn.t͡ɕā.ná(ʔ).bū.rīː]) is the largest province (changwat) of Western Thailand. Tourists are attracted by the history of its ancient civilization and the World War II Bridge over the River Kwai, originally spelt "Khwae" but officially changed to Kwai to accommodate the expectations of tourists.

Archaeological remains found in Kanchanaburi date back to the 4th century, with evidence of trade with surrounding regions at that time. Very little is known about the historical Khmer influence in Kanchanaburi, but Prasat Muang Sing, one of the country's most well-known Khmer sites, provides evidence of their occupation.

Not much was historically recorded about Kanchanaburi province before the reign of King Rama I, but some historians believe that the province was of strategic importance during the Ayutthaya period, since it was on the invasion route from Burma. In 1982, many human and elephant skeletons and swords were found in Phanom Thuan district, leading to speculation that this site might even have been the site of the famous battle of King Naresuan against the Burmese crown prince, most commonly assigned to the Don Chedi district in nearby Suphanburi province.

Most foreigners are mainly aware of Kanchanaburi's recent history with the Burma Railway. During the Japanese occupation of Thailand in 1942, both allied POWs and Asian labourers were ordered by the Japanese to build a Thailand-Burma railway. Eventually, more than 100,000 people (16,000 allied POWs and 90,000 local Asian labourers) died from horrific working conditions.

The province is in the west of Thailand, 129 km from Bangkok, and covers a total area of approximately 19,482 km² (7,522 sq mi). It is the country's third largest province, after Nakhon Ratchasima and Chiang Mai. Topographically, it is covered with timber and evergreen forests. The total forest area is 11,990 km² (4,630 sq mi) or 61.5 percent of provincial area. The district covers the source valleys of the rivers Kwae Yai and Kwae Noi ("River Kwai"), which merge at Kanchanaburi city to form the Mae Klong River.
The neighboring provinces are (clockwise, from the north) Tak, Uthai Thani, Suphan Buri, Nakhon Pathom, and Ratchaburi. In the west it borders Kayin State, Mon State, and the Tanintharyi Region of Myanmar.
Bong Ti is a transnational border crossing point, which is expected to gain in importance if the planned Dawei deepwater port project goes ahead, along with a highway and a railway line between Bangkok and the port.

Seven of the nine national parks in the mountainous areas of the Tenasserim Hills from region 3 (Ban Pong), Phu Toei and Thai Prachan national parks are not in Kanchanaburi province, form protected areas in Kanchanaburi province.(visitors in fiscal year 2024)

Two of three wildlife sanctuaries from region 3 (Ban Pong), Mae Nam Phachi is not in Kanchanaburi province, are the protected areas of Kanchanaburi province.

Kanchanaburi province has a tropical savanna climate (Köppen climate classification Aw). Winters are dry and very warm. Temperatures rise until April, which is very hot with the average daily maximum at 37.8 °C (100.0 °F). The monsoon season runs from May through October, with heavy rain and somewhat cooler temparatures during the day, although nights remain warm.