Post Malone is about to celebrate a very big milestone ... he and his GF are having a baby!!!
Post tells TMZ ... "I'm excited for this next chapter in my life, I'm the happiest I've ever been, and for since I could remember I was sad. Time to take care of my body and my family and friends, and spread as much love as we can every day."
Sources close to Post say he and his girlfriend celebrated the baby-to-be with a private party for friends and family in Southern California over the weekend.
TMZ broke the story -- Post was super sweet to a fan with autism -- who was out celebrating his 21st. The fan's mom told us her son has a hard time speaking, and strangers are sometimes impatient with him ... but Post could not have been nicer.