William, who like his father the Prince of Wales is a passionate campaigner for animal conservation, demanded "dramatic change" in how the species is protected.
See also: Illegal wildlife trade: What is being done to stop it?
Speaking to ITV News, he condemned illegal poaching, which is decimating rhino and elephant numbers in Africa, and he agreed it is "make or break" time to save the rhino.
William said: "In the next five to ten years if there's not a massive change, a dramatic change, in the way we appreciate and protect these iconic species in Africa, there won't be these incredible animals there, which not only is obviously sad for future generations but I think it would be incredibly devastating for humanity itself when we have sat back and we have lost something we have been responsible for."
Underlining the gravity of his warning, William added: "If we haven't achieved something in the next five to ten years then it will be almost impossible to do anything after that.
"Because the numbers will be so depleted the damage will be done so badly and clearly the demand won't have been halted."
The battle to protect wild animals from illegal poaching is a campaign close to the Duke's heart.
Last October, William used a speech ahead of a state visit by China's president to Britain to urge the Chinese to stop buying ivory and horn for trinkets and medicine.
In February 2014 he appeared alongside his father in a video urging the world to "unite for wildlife" – a phrase the pair say in several languages, including Mandarin, Arabic and Vietnamese.
In the video, William said: "My father and I hope you share our belief that it is shocking that future generations may know a world without these magnificent animals, and the habitats upon which they depend."
He has repeatedly spoken about how he feels the responsibility to protect these animals for future generations all the more since becoming a father to two year-old George and ten-month-old Charlotte.