Numbers of the critically endangered mammals increased from 4,845 in 2012 to 5,630 in 2018 - a yearly rise of 2.5 per cent.
The International Union for Conservation of Nature (IUCN) says it is expecting a further slow increase over the next five years.
The rise in black rhino numbers is mainly due to increased work to stop poachers who target the rhinos for their horns.
Poaching, when people kill the animals for their horns, is still the main threat to rhinos.
The IUCN also said moving selected rhinos from established populations to new locations to increase the population has also played a part.
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