In the photo, Dunham, 32, leans against Antonoff’s shoulder, while the Bleachers frontman flashes a big grin.
“Best friends 4ever,” she wrote alongside the image.
Last week, while sharing a smiling photo of Antonoff that appeared to have been taken on June 13, she wrote that even though the pair had “broken up,” they had an unbreakable bond between them.
“Happy father’s day @jackantonoff,” she wrote. “We may have broken up but you still give me the finger from the London airport and that’s 4ever.”
Earlier this month, the friendly exes also raised a few eyebrows after exchanging some seemingly flirty messages on social media.
“Cuz [sic] it’s the prime time to catch you on the gram, four eyes,” Dunham responded.
The former couple met in 2012 after being set up on a blind date by Antonoff’s sister Rachel and comedian Mike Birbiglia.
Days after PEOPLE confirmed Dunham and Antonoff’s split, she briefly addressed the breakup during a livestream on Instagram, saying she was still wearing a ring that Antonoff had given her.
“I’ll always wear it because love is a really cool, powerful, eternal thing,” she said. “It doesn’t have to be defined the way that we, in Western culture, define it as beginnings and ends.”