The 45-year-old actress dressed up in a marching band uniform to wish Witherspoon a happy 42nd birthday Thursday.
Garner shared a video on Instagram of herself performing "Happy Birthday." The star wore a red, white and black outfit with a matching cap and pompom.
"Hey, @reesewitherspoon! I don't know why you're on my mind today..... #happybirthdayfriend," she captioned the clip.
Garner said on Thursday's episode of The Late Late Show with James Corden that she attended band camp when she was young. She shared a NSFW story about a trumpet player who was sent home after a hookup.
"I went to band camp somewhere in the hills of West Virginia," the star told host James Corden.
"We did have a 'One time at band camp' situation," she said, referencing the American Pie quote. "Oftentimes at band camp you'll have your first experience with someone, some kind of juicy experience."
"Hmmmm @reesewitherspoon I know it's your birthday, but I think we could try and pass for high school students!?! Just think about it. #happybirthday," she wrote.
Witherspoon thanked her friends and fans in a post on Instagram, saying she was "feeling so much love" on her birthday. The actress started filming Season 2 of the HBO series Big Little Lies this week.