The couple were first linked in the press in September 2011, with reports suggesting that they first 'hooked up' on the set of GaGa's music video 'You and I'.
GaGa and Kinney have even been rumoured to be tying the knot when the singer's 'Born This Way Ball' tour comes to Italy this week.
Kinney did not address the wedding rumours in a USA Today interview, but did declare that he and GaGa are very much in love.
"I don't think I've ever said anything publicly up till now, but I'm happy," the Chicago Fire actor explained.
He went on to say: "I'm in a relationship, and I'm happy. Everything is great!"
GaGa previously insisted that she will commit for life once she does marry, explaining that "divorce is not an option".
Kinney's new series Chicago Fire premieres Wednesday, October 10 at 10/9c on NBC.
Watch Lady GaGa's 'You and I' music video below: