Garner, 47, took to Instagram on Tuesday to share a gallery packed with special moments from the film, along with photos of her young cast mates 15 years ago and today.
The 2004 time-travel romance told the story of a gawky 13-year-old girl named Jenna Rink who, after being humiliated at her birthday party, dreams of being 30.
Jenna wakes up the next morning in the body an adult woman (Garner) who lives in a posh Fifth Avenue apartment and works at a fashion magazine.
Grown-up Jenna grapples with all the ups and downs of adulthood, including a burgeoning romance with the man-sized version of her platonic pal Matt (Mark Ruffalo) — before she's transformed back into a tween.
In a long, heartfelt caption, Garner gushed about the "joyful" movie's enduring appeal and praised her young co-stars — including "Captain Marvel" star Brie Larson and "Pretty Little Liars" alum Ashley Benson — who now enjoy successful Hollywood careers of their own.
"The movie benefitted from the performances of so many young actresses. Open and twinkly-magical then —now, 15 years later, they are ruling the world," she wrote.
"I loved my day with this slumber party crew," Garner continued, referring to one of the movie's most popular scenes. "They danced with abandon and reminded me of the power of just saying yes — to being in a group, to vulnerability, to wearing a bra over your clothes."
"Imagine if every 13 year old girl had an opportunity to dance and feel beautiful like the girls in this film," she added.
The "Alias" alum also noted that, over the years, "13 Going on 30" has only grown in popularity, with singer Ariana Grande paying homage to it in her "Thank U, Next" video.
"I feel so, so lucky to have had the experience of making this joyful movie. And even luckier to be part of something that, all of these years later, can still make people (hi, @arianagrande) smile," she wrote.