Alex Bregman's fiancee decided to spice up quarantine time with a neck-cracking prank on the MLB star ... and it's gnarly and hilarious all at the same time!!

The gag began with the Astros third baseman's soon-to-be wifey placing a piece of dry pasta in her mouth and calling Alex over to help her "crack her neck."

Bregman -- who, by the way, didn't want to come because he was working on a puzzle -- showed up and pressed on the side of her head.

That's when Reagan chomped down on the pasta -- making a sick cracking noise -- AND IT SCARED THE HELL OUT OF ALEX!!!

Watch the clip ... the 26-year-old 2019 AL MVP candidate jumped back several feet with a look of sheer terror on his face!!!

Reagan quickly showed him it was all part of a TikTok video ... and Alex couldn't have been more displeased, calling it "stupid!"

Don't worry ... we're sure Alex got over it -- because, well, how could you stay mad at this?!?!