The "Joker" star, who was named Best Actor for his performance in the film during the Golden Globes on Sunday (January 05), announced the news on Wednesday (January 08) and the designer quickly praised his decision.
"This man is a winner... wearing custom Stella because he chooses to make choices for the future of the planet," McCartney tweeted. "He has also chosen to wear this same Tux for the entire award season to reduce waste. I am proud to join forces with you."
Phoenix's advocacy work was also in play during the Golden Globes award show itself when he praised organisers at the Hollywood Foreign Press Association for offering an all-vegan menu during the prizegiving.
"It's the first time I've ever eaten the food at this ceremony," the longtime vegan and animal rights supporter told reporters backstage during the event. "I thought it was excellent. I was so moved by their decision to make tonight plant-based. It was such an important step... I think we're coming to a point now where the evidence is irrefutable and undeniable... I think now consuming animal products is no longer just a personal choice. It is having a drastic and vast consequence on the rest of the world and all of us."