Gerber, 18, revealed they have matching tattoos on Wednesday while wishing Delevingne a "Happy 28th Birthday" on her Instagram Stories.
"Happy birthday to my solemate," Gerber wrote as she posted a photo of their feet showing off the ink in their arches.
In another birthday tribute on her Stories, Gerber called Delevingne "the best protest buddy."
Back in July, the two friends were spotted marching together at a Black Lives Matter protest in Los Angeles. Delevingne had just split from girlfriend Ashley Benson and Gerber was previously spotted with comedian Pete Davidson during the winter.
Meanwhile, Delevingne used her birthday for a good cause.
She posted on social media asking her over 44 million followers to sign her petition to demand Congress pass the Break Free From Pollution Act.
For my birthday this year, join me in demanding Congress pass the Break Free From Pollution Act and put our planet and public health above corporate profit. 🌎 Link in my bio to sign the petition - can we get 28,000 signatures for my 28th birthday? @sierraclub."
Delevingne's other close friend Margaret Qualley said on Instagram, "Dear @caradelevingne, man, too much to say. So incredibly lucky to have you as a friend. I love you, happy birthday."
"Happy burfday bubba," wrote fellow model Joan Smalls on her Instagram Stories. "Keep on turning up and shining through."
And Georgia May Jagger said, "Happy Birthday!!!!! @caradelevingne I love you so much. Can't wait to be old dog obsessed ladies sitting on the porch with towels on our heads chatting away lol."