It was an ultimate girls-only getaway in Tulum, where the trio hit the surf for a little fun in the sun.
Witherspoon, 36, showed off her post-baby body in a modest, red-and-navy striped halter-style one-piece.
Fellow new mom Barrymore, meanwhile, tested the waters in a black-and-white polka-dot bikini with retro high-waisted bottoms and a bandeau-style top, reported Us magazine.
"Reese and Drew have bonded over their babies and losing the baby weight," a source said of the A-list actresses, who gave birth within 24 hours of each other last fall.
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The ladies were joined on the beach by Barrymore's 'Charlie's Angels' costar Diaz, 40, who showed off her famously fit physique in a skimpy two-piece with yellow bottoms and a striped top.
"She seemed excited to just be with her friends and be in a healthy, fit place where she could work out all day and relax," an eyewitness said.
The trio did yoga, ran on the beach, swam, and noshed on healthy meals for five days. "They all hung out together and seemed to be good friends. All the girls were very social and had a great bonding experience.