The Joker star has teamed up with Veganuary - a global organization encouraging people to try vegan in January and beyond - to encourage people to sign the 31-day plant-based pledge. This year, more than 250,000 people took part - with 350,000 sign-ups the goal for 2020.
He joins a host of other prolific people including Alicia Silverstone, Mayim Bailik, Evanna Lynch, Monami Frost, George Monbiot, Jason Gillespie and many others backing the initiative.
'Violence of our food system'
"If you look at the climate crisis or the violence of our food system and feel helpless, thinking 'I wish there was something I could do'- you can. Right now. Sign up to try vegan this January," Joaquin Phoenix said in a statement sent to Plant Based News.
Toni Vernelli, Veganuary's Head of Communications, added: "Each of us can help fight climate chaos with our fork.
"Signing up at Veganuary.com is a fun and empowering way to tackle this serious global threat. All of our resources are completely free, friendly and supportive - so why not do your part for the planet and give it a go!"
Veganuary is expecting 2020 to be its biggest year yet, and as part of that, is planning to screen a pro-vegan advert on mainstream TV in the US, UK, and Germany between Christmas and New Year.
The charity has launched a crowdfunder campaign to get its advert aired as widely as possible. Every donation made in the next week will be doubled as part of The Big Give's Christmas Challenge - the UK's biggest match funding campaign.
"In true Veganuary style [the advert is] fun, funny and non-judgmental, yet is guaranteed to make people question how comfortable they truly are with their food choices," says Veganuary.
'The power of TV advertising'
"We all know the power of TV advertising to capture people's attention and influence their behavior, so getting the first pro-vegan ad on TV will be a major milestone for our movement," Toni Vernelli, Head of Communications at Veganuary, said in a statement sent to Plant Based News.
"Our crowdfunding campaign gives everyone a chance to play a part in making it happen.
"Thanks to the Big Give every pound donated will have double the impact, but we only have a week to raise the £40,000 needed to get this bold, fresh, and compelling ad in front of millions of people and inspire them to try vegan this January and beyond. Your support could help make history."