Witherspoon shared a photo of her dinner date at a secret location, enjoying a glass of wine and a cake that lit up the room. "This is 40... What a lucky girl," she wrote alongside the Instagram.
She and 48-year-old Kidman go a long way back and are currently working together as producers on Big Little Lies.
The intimate meal isn't Witherspoon's only birthday celebration though—she also had a blowout party at the Warwick in Hollywood over the weekend.
In addition to Kidman and husband Keith Urban, Kate Hudson, Jennifer Ansiton, Robert Downey Jr., and Courtney Cox were among the star guests who helped the actress toast the big 4-0. Plus, Taylor Swift put in a surprise performance to get the guests dancing.
Witherspoon has also being getting plenty of love for her big day at home, with her 16-year-old daughter Ava Phillippe sharing an Instagram of her mom with half-brother Tennessee, writing: "Happy birthday to this lovely woman! Thanks for always being there to support me. Have the best day."