The Irishman shared a steamy sex scene with topless Nicole Beharie in the 2011 film and now they're an item in real-life.
And, talking about the romance for the first time, Fassbender reveals the professional couple agreed not to act on passionate feelings during the shoot and waited until the film had wrapped before they started dating.
He tells GQ magazine, "Nothing happened while we were filming. We started talking more on the promotion thing. So, yeah, it just sort of unfolded like that."
However, the British actor admits geography has hampered their relationship, adding, "We're trying to see each other as often as possible. That's kind of difficult when she lives here (in New York) and I live in England."
The 35 year old was last linked to his X-Men: First Class co-star Zoe Kravitz, who bears a striking resemblance to his new girlfriend.