SI Swimsuit model Haley Kalil -- wife of NFL offensive tackle Matt Kalil -- came up with a way to follow her mom's "no bikini" rule and still show some skin at the same time:

One-sided one-piece suit!!! ... and if you've never heard of the term, it's exactly what you think it is.

"MOM IT’S TECHNICALLY A ONE PIECE 😂😎😂," Haley said of the suit choice.

Of course, the model was kidding about the whole "no bikini rule" ... considering she's been spotted in the Sports Illustrated shoots for the past 2 years.

This shoot went down in the British Virgin Islands ... with Kalil sporting the fully-there front with some floss-thin strings holding everything together in the back.

The 2020 edition of the mag -- with cover models Kate Bock, Olivia Culpo and Jasmine Sanders -- just hit shelves on Tuesday ... and Haley's shoot in the epic suit is featured in it.

So, don't worry, mom!! Still following the rules.