The sportsman joined the British royal in London on Monday (09Jun14) to launch United for Wildlife, a campaign aimed at protecting animals from illegal poaching.
Speaking to the media, William, who serves as president of the organisation, said, "The illegal wildlife trade is responsible for the slaughter of tens of thousands of animals a year, pushing some of our most beloved species to the brink of extinction.
"Our children should not live in a world without elephants, tigers, lions and rhinos. Enough is enough. It is time to choose between critically endangered species and the criminals who kill them for money."
Prince William previously recruited Beckham to speak out against animal poaching in China for wildlife protection fund WildAid.
William's brother Prince Harry, Scottish tennis ace Andy Murray and Formula One star Lewis Hamilton have also been appointed as spokesmen for United for Wildlife.
The stars are aiming to drum up support for the cause by spreading the word via social media with the use of the hashtag #WhoseSideAreYouOn and the message: "Join us and help stop illegal poaching."