The Prince of Wales posed for selfies with locals, cuddled a baby, and planted new trees during a visit to Paqueta, an island a ferry away from Rio de Janeiro.
Prince William is in Brazil to present the Earthshot Prize, the annual award from the charity he set up, on Wednesday.
He is also scheduled to give a speech at next week's COP30, the UN's annual climate meeting where governments discuss how to limit and prepare for further climate change, the following day.
The future king is two days into his five-day visit to the country, which started on Monday with an honour from the capital's mayor, a trip to Sugarloaf Mountain, and training drills with children at the famous Maracana Stadium.
Prince William's visit is the most significant royal engagement he will make this year and also mark the first time he will be seen representing the Royal Family since the crisis surrounding his uncle Andrew.
