The expecting parents had a gender reveal at what appeared to be a family gathering. There's Kourt, baby bump front and center, sitting on Trav's lap ... with a drum set at the ready.

Then you hear a voice -- sounds like Khloe -- yelling, "Let's get the party started! We're all excited."

Barker then takes to the instrument that made him world-famous, producing a drumroll, and then striking the cymbals ... suddenly, blue streamers shoot into the sky. So, it's a boy!

Someone predicted it, screaming, "I knew it!" Travis and Kourtney hugged and kissed ... something we've now seen more than a million times.

As you know, Kourtney announced they were having a kid at a Blink-182 concert last weekend in L.A.