Prince Charles and the Titanic star chatted privately as they were given a guided tour by the designer of her sustainable exhibit at the Kelvingrove Art Gallery in Glasgow.
The future King was attending the event as part of the COP26 summit, when actor Leonardo was spotted.
WATCH: Prince Charles meets Leonardo DiCaprio and Stella McCartney
The Wolf of Wall Street star is a climate change campaigner, working on a number of documentaries about animal poaching and the environment, and was appointed a United Nations representative on climate change in 2014.
Charles and Leonardo viewed the items on display in the gallery including luxury handbags made from vegan, lab-grown mushroom leather and futuristic vegan football boots, the designer created in collaboration with Manchester United star Paul Pogba and Adidas.
Charles chatted with Hollywood actor Leonardo DiCaprio
The actor was a guest of Stella's and the trio spoke together for around ten minutes before the designer joined a live discussion with industry experts about stopping the use of animal leather and fur.
Charles learned about Stella McCartney's sustainable fashion
Fashion designer Stella, who created the Duchess of Sussex's second wedding dress, said Charles found her exhibition "enlightening and inspiring".
She added: "I want to highlight my industry and call it out, we're one of the most harmful industries in the world to the environment and what I'm doing here is trying to provide sustainable solutions and technologies and a better way of doing things."
Commenting about DiCaprio's reaction she added: "He's just like 'I can't believe you're the only person in fashion doing this' and I think he's a bit gobsmacked I don't have more colleagues joining forces with me."
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