The Wedding Crashers star unveiled her new look on Instagram writing: "Go blonde in ‘19 or go home".
The actress, who shares three children with star Sacha Baron Cohen, has previously opened up about her signature red hair, saying although she used to get teased for her natural hair colour, she's learnt to embrace it over the years.
"I got teased growing up in Australia and then in London, where people label you a ginger. It can make you feel like an outsider," she said to Men's Journal.
"Now I embrace it because it makes me unique. I also used to want to be taller and always wore heels, but then I realised the entire sight gag in Wedding Crashers worked because of the height difference between me and Vince [Vaughn]. Now I think that the more you you can be, the better. Everybody else is taken."