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.