Jessie made the announcement on IG Sunday, writing ... "Our beautiful boy is here 💙 Denver Calloway Decker," ... attaching a snap of her cradling her newborn with Eric looking over him in pure awe and wonder.

JJD gave us another snap of the sleeping younger ... who tipped the scales at a healthy 8.7 pounds upon his arrival.

The happy couple -- who are already mom & dad of daughter Vivianne and sons Eric Jr. and Forrest -- kept their news under wraps for a solid 10 days before sharing it with the world ... listing his official birth date as February 9.

Heidi Montag, Kim Zolciak-Biermann and Jennifer Love Hewitt were just a few of the couple's celeb pals who dropped congratulatory comments ... along with the rest of their fans who gave a big thumbs up to the lil one's name choice. Denver's a massive hit!

Jessie's pregnancy was announced in August ... and caught everyone by surprise, including Eric, who'd been considering getting a vasectomy after their third -- but he never went through with it.

Jessie spilled the tea in an IG Q&A shortly after the big reveal ... sharing that Eric was so shocked he thought she was pulling a prank on him. In fact, he was convinced her then-pregnant sister had peed on the stick to fake her own results.