The ex-Prez made the announcement Monday -- writing ... "J.D. honorably served our Country in the Marine Corps, graduated from Ohio State University in two years, Summa Cum Laude, and is a Yale Law School Graduate, where he was Editor of The Yale Law Journal, and President of the Yale Law Veterans Association. J.D.’s book, 'Hillbilly Elegy,' became a Major Best Seller and Movie, as it championed the hardworking men and women of our Country."

He continues ... "J.D. has had a very successful business career in Technology and Finance, and now, during the Campaign, will be strongly focused on the people he fought so brilliantly for, the American Workers and Farmers in Pennsylvania, Michigan, Wisconsin, Ohio, Minnesota, and far beyond ..."

Finally, Trump ended with this ... "As Vice President, J.D. will continue to fight for our Constitution, stand with our Troops, and will do everything he can to help me MAKE AMERICA GREAT AGAIN. Congratulations to Senator J.D. Vance, his wife, Usha, who also graduated from Yale Law School, and their three beautiful children. MAGA2024!"

The news comes ahead of the beginning of the Republican National Convention in Milwaukee -- where Trump is in town ... and where he's scheduled to give a speech on Thursday.

Of course, it also comes on the heels of 20-year-old Thomas Matthew Crooks trying to shoot and kill Trump at a rally in PA this past weekend -- which DT miraculously survived.

The media seems to have gotten tipped off earlier Monday morning that J.D. would be Trump's pick for VP ... 'cause footage has surfaced showing him leaving his Cincinnati home for the RNC in a full-blown motorcade and a ton of security at his place.

J.D. is one of Trump's most loyal allies -- and he strongly spoke out on the assassination attempt -- seemingly blaming it all on Joe Biden's campaign shortly after the incident.

His support is a big shift from 2016 when he called Trump "America's Hitler." But during the 2022 Senate run, he deleted those controversial tweets and credited Trump for his accomplishments in office.

At 39, Vance is much younger than Trump's former VP Mike Pence ... so this marks a fresh direction for the ex-President, and could potentially open him up to new voters.

It's been full-steam ahead for Trump and co. despite the assassination attempt -- he's signaled that he won't be deterred ... and now, he's got his right-hand man to help make the final sprint toward the White House in November.