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 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.