Indonesia's highest Islamic authority has said it is a sin for people to deliberately burn forests to clear the land for growing crops.
Illegal slash-and-burn farming has devastated large areas of Indonesia and causes air pollution which affects countries around the region.
Indonesia has repeatedly been accused of not doing enough to stop it.
Government officials said they hoped the moral impact of the fatwa would help reinforce laws against it.
At a joint conference in Jakarta, the Indonesian Ulema Council (MUI) and the Indonesian environment ministry said they were in talks about how to enforce it.
Hayu Prabowo, head of environmental protection at MUI, said most of the forest burning was done by companies, but declaring it haram - forbidden by Islamic law - should make the public feel empowered to stand up to them.
"This fatwa will make religious leaders and the general public who in the past didn't care, care and start to take responsibility to end forest burning in their area," he said.