The bodybuilder had a gym-themed birthday bash and celebrated with his friends and college sweetheart, Nicky Dodaj, whom he met at his alma mater, Pepperdine University.
The pool party, titled “Joe’s Gym,” had guests enjoying throwback decorations and trying out workout recovery equipment. Shots were given to Baena via toy water guns.
"JOE’S GYM OPEN FOR BUSINESS!" Baena captioned the slideshow of photos from his pool party.
His girlfriend also posted a sweet tribute to her love on his birthday.
"Happy birthday to my person !!!!! From group partners in a business project to now !!! I love you & can’t wait to celebrate with you," she wrote.
Baena, who is often compared to his famous father, is becoming a fitness guru, often posting his fitness regime and his progress regarding bodybuilding on his Instagram.
He also has posted with his famous bodybuilder father, Arnold.
Baena made headlines on the Internet for recreating his father’s iconic bodybuilding photo on Instagram. The original photo, which Baena’s pose was clearly inspired by, was taken right after Schwarzenegger won Mr. Olympia for the sixth straight time in 1975.
Schwarzenegger, 72, had son Joseph with Mildred Baena, his former housekeeper while he was still married to Maria Shriver. He shares four children with his ex-wife: Katherine, 29, Christina, 28, Patrick, 26, and Christopher, 22.
Fox News' Jeremy Nifras contributed to this report.