We all know what Bill Belichick's wearing on Super Bowl Sunday (hoodie) -- but his girlfriend Linda Holliday is going with A LOT more flair ... with a custom crystal outfit, and TMZ Sports has the pics!

Linda hit up Leah Miller at Bella Artistry who makes custom Swarovski crystal fashion items for a TON of athletes and their significant others ... and ordered up some fresh gear for SBLII.

Holliday will be wearing a special blazer with her BF's last name spelled out across the back and his initials on the arm. We're told the blazer has 3,500 crystals that were all hand-placed, one-by-one.

She also got a blinged out purse complete with 1,200 Swarovski crystals!

Bill's boss is a fan of Leah's work as well -- Robert Kraft is getting a special crystalized pair of Nikes ... customized for the Pats owner himself with 13,000 crystals!!!