Justin’s representative told People magazine: “Justin Theroux had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage.”
Jennifer’s publicist also confirmed the engagement but no wedding date has been set yet.
The couple have been friends for years but only started dating after filming the movie Wanderlust together.
This will be 43-year-old Jennifer’s second marriage; she was previously wed to Brad Pitt for five years until they divorced in 2005.
Jennifer’s engagement announcement comes four months after Brad and his long-term partner Angelina Jolie – who he raises six children with – announced they are to marry.
Justin previously dated Heidi Bivens for 14 years before beginning a relationship with Aniston.
While Jennifer prefers not to comment on her romantic relationships, she has previously called Justin her “protector” and admitted he has had a massively positive impact on her life.
She said: “He’s a protector, for sure. He’s just a good human being and so funny.”
Jennifer recently bought a $21 million home in Beverly Hills for the couple to live in.
The property, which comes with its own vineyard, is currently being refurbished before they move in.