Archaeological evidence shows that long before the great Lao kingdom of Lān Xāng (second quarter of the 14th- end of the 17th entry) was established, several forms of Buddhism were practiced in the territory of the current Lao People's Democratic Republic and parts of Northeastern Thailand (Lorrillrad 2008, 2017a). Local historiography reflects in its own way the antiquity of these practices, bringing Buddhism from Cambodia in the middle of the 14th century. Numerous testimonies reveal, however, that during its golden age (mid-15th to mid-16th centuries), Lān Nā's religious culture exercised a masterly influence over the middle Mekong Valley (Lorrillard 2003, 2017b). The first Lao inscriptions, little studied so far, reveal extremely significant data on this subject. They not only change our view of the type of Buddhism that was practiced in Lān Xāng from the beginning of the 16th century, but also helps us understand the spread of a new form of religion in a space already strongly imbued with the memory of ancient civilization.