The two were first rumoured to be romantically involved last spring as they filmed director David Wain's upcoming comedy Wanderlust.
Aniston discussed her and Theroux's relationship in a Good Morning America interview and revealed that she was friends with the actor long before they began dating.
"Nothing happened on [Wanderlust]. I mean, that was sort of - that's what is a rumour. I mean, it's again, it's the easy rumour," the star explained. "Nothing happened. We were just friends, great friends, on the movie."
She continued: "I've known Justin for a long time… we met when he was shooting Tropic Thunder [years ago]… He is really cute."
Aniston also commented that she loved reuniting with her Friends and The Object of My Affection co-star Paul Rudd for Wanderlust.
"I mean, he's just the easiest person, the most fun. He's so funny and sweet and excited," Aniston said. "And you know, he's just a great guy. He's a great guy. And he is a great partner. And he really was fun, it's always fun to work with him."
Wanderlust - which also stars Lauren Ambrose and Alan Alda – opens on Friday, February 24 in US cinemas and on March 2 in the UK.
> Jennifer Aniston 'had the time of her life making Wanderlust'
> Jennifer Aniston to receive Hollywood Walk of Fame star
Watch the Wanderlust trailer below: