Giannis Antetokounmpo and his longtime girlfriend, Mariah Riddlesprigger, have welcomed their baby boy ... and his name is Liam Charles Antetokounmpo!!

The Milwaukee Bucks superstar posted the announcement on Twitter Monday afternoon ... saying their baby is "in the house."

The Alphabet also shared a glimpse of his son -- but with a baby emoji covering his face.

Of course, Giannis and Mariah -- a former Rice University volleyball player -- have been together for years ... and this is their first child together.

As for Giannis' on-the-court business ... the dude is crushing it this season after winning the MVP trophy in 2019. He's currently averaging 30 points and 13.5 rebounds a game.

So, yeah ... this year has been good to Giannis.