The actor proposed to the former Friends star on Friday, his 41st birthday - and was delighted when she said yes.
In a statement to People.com, his representative says, "Justin Theroux had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage."
The news comes just days after it was alleged in U.S. tabloids that the stars, who began dating last year, had split.
This will be Theroux's first marriage, while Aniston was previously wed to Brad Pitt.
They split in 2005 after four-and-a-half years of marriage, and the Ocean's Eleven star is now engaged to Angelina Jolie.