The blond actress went bowling with her beau Garrett Hedlund and a group of six friends at Pinz bowling center in Studio City earlier this week.
A source tells E! News that KiKi's birthday crew arrived around 8 p.m. and stayed for more than two hours.
"The whole group seemed like they had a really good time," our insider told us.
Dunst and Hedlund couldn't keep their hands off of one another throughout the birthday bash and showed off some major PDA in between their turns.
The close knit group of friends all sang "Happy Birthday" to the lady of the hour and enjoyed pizza during the low-key evening outing.
The eyewitness added that Dunst "was very happy and cheerful" and that Hedlund turned out to be a hot shot when it came to his bowling skills.
By the end of the evening, the hunky actor had the highest score and was proud of his accomplishments.
Dunst continued, "We all have to get our own jobs and make our own money, but staying at home, nurturing, being the mother, cooking—it's a valuable thing my mom created.
"And sometimes, you need your knight in shining armor. I'm sorry. You need a man to be a man and a woman to be a woman. That's why relationships work."