Prince Albert has said he's "saddened" after learning of the death of his cousin aged just 29.
The Monaco Prince and his wife, Princess Charlene, paid tribute to his second cousin Antoine de Massy, the grandson of Princess Antoinette, the elder sister of Prince Rainier III.
Antoine, an extended member of the family was a keen tennis player. He was the son of Christian de Massy, Albert's first cousin.
The royal couple said in a statement on Monday: "Their Serene Highnesses Prince Albert II and Princess Charlène were saddened to learn of the death of Antoine de Massy, aged 29, the grandson of Princess Antoinette, the elder sister of Prince Rainier III.
"A keen tennis player, he was part of a family history closely linked to the sport in the Principality.
"Their Highnesses share in the grief of his family and loved ones and offer them their sincere condolences at this painful time."
Little is known about the circumstances surrounding Antoine's death at this time, though the Nice Matin reported he was found in a public garden in Monaco.
According to the French outlet, an individual is being held in custody. The possibility of an accident has not been ruled out at this time.
All about Prince Albert's relative Antoine de Massy
Antoine was also the grandson of Anthony Noghès, founder of the Automobile Club de Monaco, and the nephew of Alexandre-Athenase Noghès, a famous Monegasque tennis player.
Antoine was passionate about tennis and started playing at the age of just three, leaving Monaco for Miami to pursue the sport with his father.
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"It was a difficult choice, but it's my choice: my father made it very clear to me that I had to give it my all from the start, and that once I made my decision, there was no going back," he told Vogue Italy in 2012.
In the same interview, he opened up about early starts for training at 8 am, and said he hoped to become the "number one" player in the world and wanted to represent Monaco at the Olympic Games.
Prince Albert and Princess Charlene's family
Prince Albert, 62, is the son of Grace Kelly and Prince Rainer III.
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He married Charlene Wittstock, 48, a former South African Olympic swimmer, in 2011.
The couple shares 11-year-old twins, Prince Jacques and Princess Gabriella.
The royal couple's most recent public engagements include inaugurating a new Ida nursery and meeting Pope Leo XIV in Metz, northern France, in October.