"I'm so in love and we've got amazing things going on," Ross, 25, said at Monday's premiere of The Hunger Games: Catching Fire.
"And we inspire each other," the actor told omg! Insider.
He revealed that he'd known the 29-year-old for a while and that their relationship had developed naturally, admitting, "She's the one."
Ross, who's the son of Diana Ross and will appear in the upcoming The Hunger Games: Mockingjay Parts 1 and 2, also said that he can see wedding bells in their future "for sure."
Simpson and Ross have been getting closer since taking their relationship public in July. The pair celebrated together with family and friends at Simpson's birthday party in October and attended his mother's Los Angeles concert in August.
Ross has even bonded with Simpson's 5-year-old son Bronx Mowgli from her previous marriage to Pete Wentz.
Be among the first to know who will be crowned PEOPLE’s Sexiest Man Alive this year! Sign up below for our daily newsletter to find out about sexy celebs and other special offers.