Fewer than 3,000 of the animals are left in the wild and environmentalists warn they could be extinct within three decades unless steps are taken to protect them.
Mukhtar, a ranger with Fauna-Flora International, says the 18-year-old elephant was not yet dead when she was found Monday in Aceh province.
Rangers unsuccessfully tried to save her by giving her medicine.
Mukhtar said it is not uncommon for elephants to be poisoned. As forests disappear, elephants stray into inhabited areas in search of food. Villagers trying to protect their property sometimes leave fruits laced with cyanide.