Simpson was first rumoured to be dating the Boardwalk Empire star in June 2011 but they have now decided to end their relationship, Us Weekly reports.
However, a source claimed that the split was amicable.
"There was no bad blood and no third party involved," the insider said. "It was honestly just a long-distance relationship and it became too hard to make it work.
"It's not necessarily forever and they could get back together. Nobody is to blame. Just the distance."
Simpson was previously married to Fall Out Boy star Pete Wentz and the pair have a 4-year-old son together, but they divorced last year.