They say an eight-year-old female in a Sumatra wildlife reserve is due to give birth to a calf in May.
The calf will be the fourth Sumatran rhino to be born in captivity, and the first in Indonesia.
The number of Sumatran rhinos has halved in the past 15 years. There are now an estimated 200 in the wild.
Widodo Ramono, of the Rhino Foundation of Indonesia, said it had been "very, very good news" when an ultrasound scan had revealed Ratu was carrying a calf.
Ratu's mate, Andalas, was born in Cincinnati Zoo in the US in 2001 and moved to the Sumatran Rhino Sanctuary at the age of six.
It is believed that when the two first met, Andalas chased Ratu, who was born in the wild, and fought with her, leaving her with serious injuries.
The smallest of the world's five rhinoceros species, the two-horned, hairy, forest dwelling Sumatran rhinos are solitary animals, often only approaching each other to mate.
The BBC's Karishma Vaswani in Jakarta says the rhinos have long been the target of poachers who seem them as valuable prizes.
There is a long-held belief their horns have medicinal properties, especially in traditional Asian medicine, says our correspondent.
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