In August 1967, five countries formed the Association of Southeast Asian Nations, or Asean, by virtue of a Bangkok Declaration. Indonesia, Malaysia, the Philippines, Singapore and Thailand were the founding members. Brunei Darussalam joined in 1985, and during the 1990s, Asean doubled its membership with Vietnam, Laos, Myanmar and Cambodia. The 2022 Asean Summit in Cambodia agreed to admit East Timor or Timor Leste as the 11th member of Asean.

The 1967 Bangkok Declaration stated as one of its aims the acceleration of economic growth and promotion of matters of common interest, inter alia, economic fields. Although no mention was made of cooperation on the environment, world events culminating in the Stockholm Declaration of 1972, which called for regional cooperation on the environment, led to the subregional Asean Environment Program beginning in 1978.