Sarah Michelle Gellar, Freddie Prinze Jr. Celebrate Bloody Bond at 'I Know What You Did Last Summer' Premiere

Sarah Michelle Gellar and Freddie Prinze Jr. made a rare joint appearance for the premiere of the "I Know What You Did Last Summer" sequel last night ... celebrating the horror franchise that brought them together back in the late '90s.

A source close to the couple tells TMZ ... It was a super special night for them -- 'cause not only does Freddie reprise his role in the highly-anticipated sequel, but it was a full circle moment for their family, as their two teens accompanied them to the premiere!

We hear it was a great night for the family celebrating the flick that means so much to them ... even if SMG's character was -- 28-year-old spoiler! -- brutally killed off in the original movie.

The longtime couple was all smiles as they walked the red carpet at Los Angeles' United Theater on Broadway hand-in-hand. The 'Buffy' star dressed to the nines in an Oscar de la Renta strapless butterfly-inspired mini-dress, while the "She's All That" heartthrob complemented her in a navy blue suit.

The Hollywood power couple met on the set of the first "I Know What You Did Last Summer" ... but they didn't start dating until 2000, and then got married in 2002. Most fans know they've worked together a few times since their slasher meet-cute ... including on the "Scooby-Doo" films.

Of course, Freddie isn't the only 'Know What You Did' OG back for the legacy sequel ... Jennifer Love Hewitt reprises her final-girl role of Julie James -- and she's been teasing in interviews that the flick is going to have a crazy twist.