"Our marriage has been tainted with my infidelities," the actor admits to PEOPLE. "I just want to be open and honest. I was irresponsible."
The actor, 40, is coming forward, he says, because a former mistress, whom he declines to name, contacted an attorney and threatened to contact media outlets.
"I was associated with a woman who I was involved with and had a relationship with," he says. "She asked for money. I felt as though I was being blackmailed or there was some sort of extortion."
Now, with the news of his infidelities out in the open, Boreanaz is getting down to the difficult task of salvaging his relationship with his wife.
"We're working on our marriage," he says. "We're working on repairing what has been damaged so badly."
To read more of the exclusive interview with Boreanaz and his wife, Jaime, pick up this week's issue of PEOPLE, on newsstands Friday