Post Malone is getting credit for giving one single mom a "Christmas miracle" ... tipping the unsuspecting server thousands of dollars after crossing paths with her during the holiday.

Here's the skinny ... the rapper stopped by The Railyard Bar in Houston on Christmas Eve, where single mom Renee Brown was working as a bartender.

As Renee told Music Mayhem ... she was having a Blue Christmas, as she was "so sad" to not spend Christmas Eve with her 9-year-old daughter. However, Post Malone rolled in with some friends, including Shaboozey, and drank for a while ... and afterward, the "Sunflower" artist tipped Renee $20,000 -- which she said changed her life at an extremely difficult time.

She added ... "This put me in a position to at least be able to get a way back and forth to work and pay up on rent and save a bit. I’m trying to save up to start a business, so I don’t have to continue working two jobs and be able to spend more time with my daughter."

Giant gratuity aside, Renee had nothing but good things to say about Post, too ... praising the hitmaker as being the "most humble, personable celebrity" she's ever had the chance to meet.

She continued ... "This was truly a blessing that I can’t fully put into words. The visit alone was absolutely amazing and something I could talk about and remember for the rest of my life."

Post certainly had reason to be generous during the holiday season ... his 2024 album, "F-1 Trillion," went certified platinum, and he had a number of chart-topping hits, including "I Had Some Help" and "Pour Me a Drink."

We'd be feeling holly and jolly, too!!!