The couple met on the set of Gossip Girl, in which Stan occasionally appears as Carter Baizen, and reportedly began dating in 2008.
However, a source told Us Weekly that they have now decided to end their relationship.
"They are still friends," the insider added. "It was amicable. They still talk all the time."
Earlier this week, their fellow Gossip Girl stars Ed Westwick and Jessica Szohr denied rumours that they had split up.