The Westworld star wrote “casual” alongside the image.
Earlier this week she teased her role will be a significant one, telling Radio Times magazine: “I’m the first woman of colour to have a prominent role in the Star Wars legacy.”
She added: “There have been others with one line and Lupita Nyong’o was a computer-generated character (The Force Awakens and The Last Jedi), but you didn’t get to see the colour of her skin.
“I’m the first. I’m going to have a toy and everything.
“It’s exciting, but that’s all I can say. It is a big deal.”
Newton will be joined by Alden Ehrenreich, Woody Harrelson, Donald Glover and Emilia Clarke in Solo: A Star Wars Story, which is due to be released in the UK in May.
Thandie Newton teases her mysterious Star Wars character Val