"Suci was a symbol of hope for her entire species, one that is quickly losing ground in the wild, and her absence will leave a hole in our hearts," Terri Roth, director of the zoo's Lindner Center for Conservation & Research of Endangered Wildlife, said in a statement.
Suci was born in 2004 and was one of three Sumatran rhino calves born at the zoo to mother Emi and father Ipuh. Emi died in 2009 and Ipuh in 2013.
Keepers had hoped to mate Suci with her younger brother, Harapan, who is now the only Sumatran rhino in North America. Andalus, the other male born at the zoo, was sent to Sumatra in 2007 to bolster a breeding program there and has fathered a male calf.
Cincinnati Zoo staff had to wait for Harapan to reach breeding age, but Suci began losing weight several months ago and staff began treating her for hemochromatosis, also known as iron storage disease.
Zoo scientists, keepers and veterinarians had been treating Suci with a therapy used on humans and African black rhinos and her behavior and appetite had improved. But her condition rapidly deteriorated Sunday, Roth said.
Zoo officials could not comment yet on their breeding plans for the future, although they say they remain committed to working to save the species.
"We are just trying to get through this right now," zoo spokeswoman Tiffany Barnes said.
The zoo has been a pioneer in captive breeding of the species, and Roth said last year that there was a lot of urgency in getting Suci pregnant.
"If we don't act quickly and boldly, the loss of this magnificent animal will be among the great tragedies of our time," Roth said.