William shared the stage with four Brazilian youngsters who each made a declaration for a world they wanted to see - from one where "we can all breathe clean air" to a future where "rivers run clear, and wildlife thrives".
Mission: Impossible star Tom Cruise, who described William as his "dear friend", congratulated the winners and finalists, thanking them for "reminding us what's possible when commitment meets purpose" in a video message to the event in Brazil.
Pop princess Kylie Minogue was one of a host of stars alongside Canadian musician Shawn Mendes and Brazilian musician and activist Gilberto Gil who performed at the awards ceremony.
Kylie praised the finalists, describing them as "amazing people doing incredible things".
The annual event, now in its fifth year and staged in Rio de Janeiro, aims to recognise and scale up solutions to "repair" the planet, and five winners were awarded Stg1 million ($A2 million) each to develop their ideas.
William told guests, including UK Prime Minister Keir Starmer: "Since we launched The Earthshot Prize, so much has been achieved.
"Your impact can be measured in our oceans, our skies, and throughout the natural world.
"I understand that some might feel discouraged in these uncertain times. I understand that there is still so much to be done.
"But this is no time for complacency, and the optimism I felt in 2020, remains ardent today.
"Because our finalists are a shining example of how we can build stronger economies, design healthier cities, and create better jobs for the next generation."
Among the winners was Lagos Fashion Week, which won the build a waste-free world category for helping to transform the green actions of the continent's fashion industry by requiring every participating designer to demonstrate a commitment to sustainable practice.
Rio de Janeiro-based project re.green won the protect and restore nature category for its work restoring tropical forests through the use of drones and satellite imagery and other data to identify suitable land.
One of the children, who were all applauded when they came to the stage, said in Portuguese: "We believe in a world where nothing is wasted, and forests stretch endlessly, rivers run clear, and wildlife thrives alongside us."
The prince will join Starmer later on Thursday at the COP30 climate summit in the city of Belem in the Amazon.
22°C