"Philly raised little man will be!" Harper said Monday.

Bryce has had the best month of his baseball career BY FAR ... 'cause after signing a 13-year, $330 million deal with Philly, he launched two massive home runs opening weekend.

But now, dude has even MORE to celebrate ... 'cause he says a baby boy with his wife, Kay Harper, is on the way -- and she says the kid will be in a Phillies uniform by THIS summer!!

"Wrigley’s gonna be pissed..." Kay said, referencing the couple's dog ... "Baby Harper making HIS debut Aug. 2019 #littleman"

Bryce and Kay have been together for years ... and tied the knot back in 2016 in an epic ceremony at one of the most famous Mormon temples in the world.

In fact, Kay reportedly played a big role in Bryce's free agency this offseason ... with the outfielder saying he wanted a long-term deal to make sure his family had roots in place for the future.

Now, dude's got that ... and just about everything else!!!