The former "Friends" star got emotional Monday night during an interview on the E! late-night talk show "Chelsea Lately."
"I just got verklempt," Aniston said as she wiped tears from her eyes after Handler, a real-life close friend of the star, noted, "You are so happy."
Theroux, 41, popped the question to Aniston, 43, in August on his birthday while the couple dined in New York City. The pair met on the set of the movie "Wanderlust" and reportedly began dating in May of last year.
Aniston had long played coy about the relationship, telling " Good Morning America" lifestyle anchor Lara Spencer in an interview last summer only that she was "extremely lucky" and "extremely happy."
Appearing as the first guest in Handler's new studio Monday, Aniston even donned her reportedly 10-carat diamond engagement ring.
"He's the greatest guy ever," Handler told her friend. "You guys make the greatest couple ever."
Theroux has had a handful of small roles in major films, including "American Psycho," "Mulholland Drive" and "Miami Vice." In addition to acting, he is also the writer behind "Tropic Thunder," "Iron Man 2," and the movie version of "Rock of Ages."
Despite Aniston's close relationship to Handler, she did not spill any details about the wedding or a date.
The actress did, however, clear up any lingering whispers about her recent headline-making commercial for Smartwater in which she showed off a faux baby bump and had Jimmy Kimmel make a cameo as her secret son.
"It was a video, honey, it's not real," Aniston told Handler.