"We are again overjoyed to experience this wonderful day in our lives together," they said in the post, which featured a photograph of the baby wrapped in a blue blanket and wearing an outfit featuring a photo of the couple's beloved dog Decoy.
A second photo showed a baby's feet peeking out from under a blanket.
"Thank you for being born safely," the proud parents wrote. "We would also like to express our heartfelt gratitude to everyone who has supported us throughout this journey."
Four-time Most Valuable Player Ohtani had not announced he was expecting a second child, but the news was out when the Dodgers announced on Friday that he was away from the team "on paternity," missing the first game of a three-game series against the Orioles in Baltimore.
However, Ohtani was not placed on Major League Baseball's paternity list and was expected to rejoin the Dodgers over the weekend.
Ohtani and Tanaka welcomed their first child, a daughter, in April of last year.