The "Star Trek" veteran and Dillon, who have kept their romance out of the spotlight for a year, were hand-in-hand at Monday's celebrity-laden premiere of "Ladies in Lavender" in London, Hellomagazine.com said.
Stewart, known as Capt. Jean-Luc Picard of the Enterprise, met Dillon when the two actors appeared opposite one another during the summer of 2003 in Ibsen's "The Master Builder," about a man who falls in love with a girl young enough to be his granddaughter.
The two reportedly have been inseparable since October 2003, when Stewart left his wife of three years, "Star Trek" series producer Wendy Neuss, and Dillon split with her boyfriend.
"Patrick is a brilliant, wonderful, gorgeous man. Some think the difference in our ages says something about him, or us, or me. But to us it's never been an obstacle," Dillon once said about Stewart.