Winslet recently joined the King at the charity's awards ceremony at London's St James's Palace, where she told him: "Don't worry, I'll be all in - I've got your back."
After the announcement, the British star said: "One of my passions is exploring ways that we can protect and promote the natural world - something I have in common with His Majesty."
Winslet added the foundation's work protecting the planet and encouraging young people to learn about the "beautiful" countryside was "deeply important" to her.
She said she was looking forward to collaborating with the other ambassadors on how to make a difference.
Other famous names who have signed up as ambassadors include UK TV stars Sarah Beeny, Patrick Grant, George Clarke, Alan Titchmarsh, musician and presenter Jools Holland.
The King's Foundation, previously known as The Prince's Foundation following an amalgamation of Charles's charities in the year he turned 70, aims to "advocate for the change His Majesty wants to see in the world".
It works to train the next generation of young craftspeople in heritage and endangered skills such as millinery, embroidery and woodworking, but also focuses on sustainability, farming and agriculture, health and wellbeing, and architecture and urbanism.
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