The 22-year-old model made the announcement not-so-subtly on IG Monday with a quick, fun video of her in a pool somewhere in the desert. She flashes the giant rock on that finger ... and flirtatiously floats away from the cameraman, presumably her fiancé.

That would be one Louis Thornton-Allan -- apparently an actor himself -- whom she just went Instagram official with last month. It's unclear how long they've been romantically involved before that ... but in any case, it looks like they're ready to walk down the aisle together.

Besides the obvious celebratory sentiment, the other feeling here is ... man, does time fly. It seems just like yesterday when she was 14, right around the time her dad tragically died in a car accident in 2013.

Of course, they were close ... and Paul's death was hard on Meadow. But, she's carried on well -- and has even shown up to a few of 'Fast & Furious' premieres since his passing. She continues to hype up PW's legacy to this day ... always promoting 'Fast' content, too.

BTW, Louis seems to have popped the question here on a special day in and of itself -- seems today's the birthday of a family member of his ... possibly his pops, 'cause he gave him a shoutout online -- in addition to posting more about his and Meadow's baecation.