The actress unveiled the new look while filming new movie Still Alice alongside Julianne Moore.
And she updated her fans on Instagram, posting a selfie to show off her new look. "Strawberry Fields #StillAlice," she captioned the snap of herself with her new red locks falling in front of her face.
It's not the first time Kate has changed up her hair colour for a role. In 2010, she dyed her hair strawberry blonde for the cowboys vs aliens movie The Warriors Way.
Speaking at the time, Kate said, "I wanted her to be a redhead, and I was hellbent on that.
"I was like, 'How can you have this fierce, fiery, crazy, feral woman in the middle of a desert with jet black hair?' I love jet black hair, but she has to be a redhead."
She also dyed her hair brunette for Superman in 2006 and Wonderland in 2003. "I dyed my hair for Wonderland," she said in an interview in 2013. "I love being a brunette and I really enjoy playing with hair colour. But dye is just so hard on your hair."
Kate recently revealed the meaning behind her bespoke Art Deco-style wedding band. Kate, who married her longtime love Michael Polish in September of last year, chose a glittering platinum double banded wedding ring, set with 12 horizontal rectangular diamonds.
"The design of the wedding band is based off the design of the engagement ring, which is a 1920s Art Deco design," said Kate, who has her own personalised jewellery company called JewelMint. "We wanted to create a wedding band that incorporated 12 of these diamond baguettes for each month of the year."