The Oscar-winner shared a snap of herself and a pal at a bachelorette party, with the caption confirming the split.
She wrote, "An about to be married woman and me, a single lady."
She went on to address her heartbreak over her recent breakup.
"I might be in a lot of pain, but I couldn't miss my best friend's bachelorette dinner," she wrote in another photo.
Gaga was looking into the camera, and holding a prop which said "bridesmaid".
Lady Gaga and Dan were first linked when they were spotted kissing in July.
A fellow diner told People, "Ironically, she had a table in the front near the sidewalk, so she was clearly okay with being seen."
And as for the singer's connection to Horton, the bystander explained, "She seemed very happy when interacting with the guy she was with."