"It happened a few weeks ago. He's really heartbroken," a source close to the couple said.
Seyfried, 29, fell for 37-year-old Long on Instagram in the summer of 2013.
"I thought something he said was really funny," she had said.
The couple shared a USD 1.9 million apartment in New York City.
Prior to her relationship with Long, Seyfried was linked to "Dexter" actor Desmond Harrington, and before that dated Dominic Cooper for three years before splitting in 2010.