Nadi was one of three northern white rhinos at the zoo's Wild Animal Park. No cause of death was given, but the female rhino had shown signs of old age, the zoo said in a statement.
Nadi, believed to be more than 40 years old, was brought from Africa in 1972 and was part of a group maintained at the Dv Kralove Zoo in the Czech Republic. She and another female rhino named Nola came to the Wild Animal Park in San Diego in 1989 as part of an effort to breed the species.
Although both females were late in life, zoo officials had hoped the presence of a male rhino and the large park habitat would help breeding. No calves were produced.
Northern white rhinos are considered critically endangered. Along with the two remaining at the Wild Animal Park, there are six at a zoo in the Czech Republic and as few as five believed to be roaming in the African wild, the zoo said.