According to E! News, the couple made a "mutual" decision to break up back in December.
They initially started dating back in 2012 after being set up on a blind date.
The pair have since collaborated creatively, with Antonoff receiving writing credits for multiple songs that appeared in Girls episodes and Dunham directing Bleachers' "Don't Take the Money" video.
In an essay penned for Variety last year, Dunham wrote: "It's safe to say that before Jack my life was full of far fewer sights. He showed me the importance of making political statements using your public platform. He showed me the joy of collaboration."
Revisit some of the former couple's previously shared Instagram photos below.
Just to prove I don't only insta about violence... Here is the picture that my ❤️ @americaferrera insisted on taking so we'd have evidence that this is how we dressed for a nice dinner. Our 5.5 year anniversary was yesterday. Still working hard to impress him in surprising ways. PS we independently dressed entirely in merch.