Arquette, 46, took to Instagram on Wednesday to share a rare family photo.
"I’m so proud of my daughter Coco!" he captioned the happy pic. "Have fun and be safe in high school! AND don’t grow up too fast! I love love love you!!!"
The photo finds the friendly exes beaming next to Coco, 14, who appears to be dressed in her Wednesday Addams costume for her school's production of "The Addams Family."
In July, Arquette opened up to Us Weekly about how proud he was of Coco for nabbing one of the play's starring roles. "She’s got the lead and it’s really exciting. She loves acting and singing and performing. She’s got the family genes," he gushed.
Arquette, who split with Cox in 2010 after 11 years of marriage, also has two sons with his second wife, Christina Arquette.
Meanwhile, Cox, 54, is engaged to Snow Patrol rocker Johnny McDaid.
The "Friends'" alum opened up about the pair's divorce in 2016 when she appeared on NBC's "Running Wild With Bear Grylls."
"We found ourselves leading separate lives and just coexisting and being great friends, but not having the intimacy that is so important in a relationship," she told Grylls. "I think you really need to work in a relationship, and I think that we tried. We're just really different, too. I'm the polar opposite of him."
Because they parted as friends, said Cox, there was no messy drama.
"I mean, as divorces go, I would say it's one of the better ones. For sure. We're good co-parents together," she shared.