Shohei Ohtani may be adding a two-sport athlete to the family ... 'cause he just posted to Instagram announcing that he and his former professional basketballer wife are having their first child.

The two-way baseball standout announced the news with a post to Instagram Saturday ... sharing a snap of his dog, Decoy, lying next to a baby onesie, two tiny sneakers and what appears to be an ultrasound photo obscured by a baby face emoji.

He captioned the photo, "Can’t wait for the little rookie to join our family soon!"

Tons of fans -- some in English, some in Japanese -- are offering their congrats online to the couple ... who married in a secret ceremony last year before the 2024 MLB season began.

If you don't know ... Sho is married to Mamiko Tanaka -- a former pro basketball player in Japan who played for the Fujitsu Red Wave from 2019 to 2023.

So -- whether this kid wants to go into baseball or basketball -- they've got a couple of great resources to ask.

It's been a banner year for Shohei who inked a massive $700 million contract in late 2023 ... before winning his third MVP award and first World Series title with the Los Angeles Dodgers.