Buddhism in Cambodia or Khmer Buddhism (Khmer: ព្រះពុទ្ធសាសនានៅកម្ពុជា, UNGEGN: Preăh Pŭtthôsasânéa noŭ Kâmpŭchéa) has existed since at least the 5th century. In its earliest form it was a type of Mahāyāna Buddhism. Today, the predominant form of Buddhism in Cambodia is Theravada Buddhism. It is enshrined in the Cambodian constitution as the official religion of the country. Theravada Buddhism has been the Cambodian state religion since the 13th century (except during the Khmer Rouge period). As of 2019 it was estimated that 97.1 percent of the population are Buddhists.

The history of Buddhism in Cambodia spans a number of successive kingdoms and empires. Buddhism entered Cambodia via two different streams. The earliest forms of Buddhism, along with Hindu influences, entered the Kingdom of Funan with Hindu merchants. In later history, a second stream of Buddhism entered Khmer culture during the Angkor empire when Cambodia absorbed the various Buddhist traditions of the Mon kingdoms of Dvaravati and Haripunchai.

For the first thousand years of Khmer history, Cambodia was ruled by a series of Hindu kings with an occasional Buddhist king, such as Jayavarman I of Funan, Jayavarman VII, who became a mahayanist, and Suryavarman I. A variety of Buddhist traditions co-existed peacefully throughout Cambodian lands, under the tolerant auspices of Hindu kings and the neighboring Mon-Theravada kingdoms.

Unconfirmed Singhalese sources state that Buddhism was introduced to Suvannaphum, or the 'Golden Peninsula', as mainland Southeast Asia was once referred to, in the 3rd century B.C. under the reign of King Ashoka, the great Buddhist ruler. According to these sources, two monks, Sona and Uttara, were sent to propagate the doctrine of the Master in this region following the great council of 274 B.C. held in Asoka's capital Pataliputta, India. Various Buddhist sects competed with Brahmanism and indigenous animistic religions over approximately the next millennium; during this period, Indian culture was highly influential.

The Funan Kingdom that flourished between 100 BC and 500 CE was Hindu, with the kings of Funan sponsoring the worship of Vishnu and Shiva. Buddhism was already present in Funan as a secondary religion in this era. Buddhism began to assert its presence from about year 450 onward, and was observed by the Chinese traveler Yijing toward the close of the seventh century.

Two Buddhist monks from Funan, named Mandrasena and Saṃghabara, took up residency in China in the 5th to 6th centuries, and translated several Buddhist sūtras from Sanskrit into Chinese. Among these texts is the Mahāyāna Mahāprajñāpāramitā Mañjuśrīparivarta Sūtra. This text was separately translated by both monks. The bodhisattva Mañjuśrī is a prominent figure in this text.

The Kingdom of Chenla replaced Funan and endured from 500–700. Buddhism was weakened in the Chenla period, but survived, as seen in the inscriptions of Sambor Prei Kuk (626) and those of Siem Reap dealing with the erection of statues of Avalokiteśvara (791). Some pre-Angkorean statuary in the Mekong Delta region indicate the existence of Sanskrit-based Sarvāstivāda Buddhism.^{[citation needed]} Khmer-style Buddha images are abundant from the period of 600–800. Many Mahāyāna bodhisattva images also date from this period, often found alongside the predominantly Hindu images of Shiva and Vishnu. An inscription from Ta Prohm temple in Siem Reap province, dated about 625, states, that the Buddha, Dharma and Sangha are flourishing.^{[dubious – discuss]}

The transition from Hindu god-king to Mahayana bodhisattva-king was probably gradual and imperceptible. The prevailing Vaishnavite and Shaivite faith traditions gave way to the worship of the Gautama Buddha and the Bodhisattva Avalokitesvara.