The world's last male northern white rhino has died at the age of 45.
Sudan was being treated for age-related complications that led to degenerative changes in his muscles and bones.
The Ol Pejeta Conservancy in Kenya says his condition worsened significantly and he was unable to stand up.
A veterinary team made the decision to euthanize him on Monday.
In a statement, the Ol Pejeta Conservancy and the DvÅ¯r Králové Zoo say: "Sudan will be remembered for his unusually memorable life.
"In the 1970s, he escaped extinction of his kind in the wild when he was moved to Dv?r Králové Zoo.
"Throughout his existence, he significantly contributed to survival of his species as he sired two females."
"During his final years, Sudan came back to Africa and stole the heart of many with his dignity and strength."
Sudan is seen in this undated photo supplied by the Ol Pejeta Conservancy | Image: Luas Frase
They also say his genetic material has been collected, with hopes for future attempts at reproduction through advanced technologies.
Richard Vigne, Ol Pejeta's CEO, says: "We on Ol Pejeta are all saddened by Sudan's death.
"He was a great ambassador for his species and will be remembered for the work he did to raise awareness globally of the plight facing not only rhinos, but also the many thousands of other species facing extinction as a result of unsustainable human activity.
"One day, his demise will hopefully be seen as a seminal moment for conservationists world wide".
Sudan is seen in this undated photo supplied by the Ol Pejeta Conservancy | Image: Ol Pejeta Conservancy
His death leaves just two female northern white rhinos on the planet: his daughter Najin and her daughter Fatu, who remain at Ol Pejeta.
The conservation says the only hope for the preservation of this subspecies "lies in developing in vitro fertilisation (IVF) techniques using eggs from the two remaining females, stored northern white rhino semen from males and surrogate southern white rhino females."
The poaching crisis of the 1970s and 80s - fueled by demand for rhino horn - wiped out the northern white rhino populations in Uganda, Central African Republic, Sudan and Chad.
The last remaining wild population ws made up of 20 to 30 rhinos in Garamba National Park in the Democratic Republic of Congo.
But they succumbed to fighting in the region during the 1990s and early 2000s.
By 2008, the northern white rhino was considered by most experts to be extinct in the wild.
In 2009, the last four fertile northern white rhinos - two males and two females - were moved to Ol Pejeta from DvÅ¯r Králové Zoo in the Czech Republic.
Sudan is seen in this undated photo supplied by the Ol Pejeta Conservancy | Image: Sophie Harrison
On arrival at Ol Pejeta, they were placed under 24-hour armed surveillance.
But despite the fact that they were seen mating, there were no successful pregnancies.
In early 2014, plans to introduce a male southern white rhino to the two females got underway - but again, this proved unsuccessful.
Tests later revealed that neither of the females was capable of natural reproduction, and that only one was fertile enough to conceive artificially.
The other northern white male, Suni, died of natural causes in October 2014.