The actress rang in the occasion with friends Sunday at Cox's home in Malibu, Calif. Aniston and Cox are longtime pals who co-starred as Rachel Green and Monica Geller on the NBC sitcom Friends.
Aniston's manager, Aleen Keshishian, was among those to document the party on social media. Keshishian shared a photo Monday on Instagram of Aniston posing with her friends in Cox's backyard.
"#happybirthday to my sister from another mother #jenniferaniston [love] u so much!" she captioned the picture.
Aniston also received birthday wishes from friend and fellow actress Reese Witherspoon, who posted a still from her 2000 guest appearance on Friends. The pair will co-star in a new Apple television series.
"Happy Birthday to my work sister #JenniferAniston!! Wishing you a day filled with lots of love and laughter! #HBD," Witherspoon wrote.
Aniston is best known for her role on Friends, which had a 10-season run from 1994 to 2004. She discussed the possibility of the show's revival in an interview with The Ellen DeGeneres Show this month.
"Anything is a possibility, Ellen," the star said. "Anything!"