Carys looked stunning as she celebrated the milestone age in the Ungaro pink slip dress with lace and roses famously worn by her mother in 1999.
She looked in her element while posing for snaps, including one with a boy kissing her on the cheek as she held a flower.
Her famous mom took to the comments section of the post and wrote: “You deserve it sweetheart.”
Zeta-Jones, 54, also shared a separate Instagram post that included a few pictures from when her daughter was younger.
Happy Birthday to my beautiful daughter @carys.douglas You are the joy of my life. 21 years of watching you blossom into the most incredible woman you are today is a God given gift that I never take for granted and cherish every day of my life. Thank you for being you. I love you with all my heart. Mama,” she wrote.
The Chicago star, who also shares Carys’ brother, Dylan, 23, with Douglas, 79, originally wore the slinky pink dress while hitting the red carpet at the 1999 MTV Movie Awards when she was nominated for best breakthrough performance for her role in The Mask of Zorro.