J.J. Watt Goes 'Social Media' Public with Soccer Star Girlfriend (PHOTO)

Big step for J.J. Watt ... the NFL superstar FINALLY went public with his relationship with his pro soccer star girlfriend -- by posting a couply picture on Instagram!

Obviously, this is a big deal.

Watt and pro soccer star Kealia Ohai have been dating for months -- but kept things extremely under wraps. No public outings. No Man Crush Monday or Woman Crush Wednesday posts. Nothing.

Until Tuesday ... when Watt posted a photo of the two during a recent beach outing and wished Ohai a happy birthday.

"Happy Birthday beautiful. May this be your best year yet! @kealiamae."

Watt has 2.5 million followers ... so, it's kind of like their coming out party.

By the way, Kealia also happens to be the sister-in-law of his Texans teammate, Brian Cushing -- so you can probably understand why Watt was being extra careful about how he handled the whole situation.