Justin Theroux is the ''happiest guy in the world'' following his engagement to Jennifer Aniston.

The actor popped the question to the former 'Friends' star earlier this month and sources say he's not been able to wipe the smile off his face since she said yes.

Onlookers who saw Justin dining at New York restaurant The Smile with four friends, told PEOPLE: ''He was beaming all night. Justin was laughing so loud the whole restaurant could hear him. [They were] obviously having a great time.

''He looked like the happiest guy in the world.''

Justin asked Jennifer to marry him on a romantic dinner date to celebrate his birthday at New York restaurant Blue Hill.

The couple then enjoyed a couple of days of private celebrations before Justin's rep announced the news in a gushing statement.

His publicist said ''Justin Theroux had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage.''

The couple have been friends for years but only started dating after filming the movie 'Wanderlust' together.