The royal gave a speech at the event and his opening and closing remarks were beautifully delivered in French. Keep scrolling to read parts of his impassioned speech.
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The event is to highlight the importance of a "sustainable and regenerative blue economy" for protecting the ocean.
"By gathering experts, policymakers, investors and innovators from around the world, the Forum will highlight and promote the critical role that sustainable investment, cutting-edge innovation, and forward-thinking policies play in ensuring the long-term health and prosperity of our oceans and economies alike," the website reads.
William walked a blue carpet as he entered the venue, alongside an official, wearing a signature navy suit and recycled and sustainable tie from Wilmok.
The royal spoke with Costa Rica's President Rodrigo Chaves Robles before they both posed alongside Prince Albert, Brazil's President Luiz Inacio Lula da Silva and France's President Emmanuel Macron.
The Prince made an impassioned speech about the need for change. He said: "We come together today united by our deep connection to the ocean and our concern for its safety. For many of us, it is a place where some of our happiest memories are made… where we have explored the wonders of the natural world… and we have all relied on its great abundance for our food and livelihoods.
"And yet, all too often, it can feel distant and disconnected from our everyday lives, allowing us to forget just how vital it is. The truth is that healthy oceans are essential to all life on earth."
In part of the speech, William pointed out that he remains "an optimist" regarding the change that can happen. "I am an optimist because as the Founder of The Earthshot Prize, I see the incredible examples of the ideas, innovations and technologies that are harnessing the power of the ocean whilst protecting its vitality," he told the attendees.
As part of the conclusion of the speech, William quoted Sir David Attenborough, and said: "If we save the sea, we save our world."
The royal decided to address the room in French for the beginning and end of his speech, which translated to: Hello. It's a pleasure to be with you today to discuss an important topic that is close to my heart." and then: "Thank you again for having me here today. Now it's time to take action. Have a great afternoon. Thank you."
He is pictured here rehearsing alone ahead of his speech.
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