Aniston's rep, Stephen Huvane, confirmed the engagement Sunday night to The Associated Press. It was first reported by People.com.
Forty-one-year-old Theroux - whose screenwriting credits include "Tropic Thunder," ''Iron Man 2" and "Rock of Ages" - popped the question on Friday, which was also his birthday.
The two both appeared in the comedy "Wanderlust," also starring Paul Rudd, earlier this year.
This will be 43-year-old Aniston's second marriage and Theroux's first. Her union with Brad Pitt from 2000 to 2005 - and their subsequent divorce - became rich material for the tabloids. After her marriage ended, she was in high-profile relationships with Vince Vaughn and John Mayer.