The new couple were seen enjoying dinner at New York's Il Buco restaurant with another pair, and the unlucky-in-love actress - who has had relationships with John Mayer and Vince Vaughn and was once married to Brad Pitt - was "very animated" while she was with her new man.
A source told Life & Style: "They sat at a corner table so they could be more secluded, but you could still see ho happy and animated Jen was. She was smiling and laughing the whole time."
The 42-year-old star - who has reportedly moved Justin in to her Los Angeles home - stayed for over an hour at the restaurant and onlookers noted they looked "very comfortable" with each other.
The source added: "Jen looked beautiful and was very happy. They just looked very comfortable together. Justin was very relaxed. They stayed for about an hour and a half."
Jennifer is said to be smitten with the 39-year-old star - who she met on the set of 'Wanderlust' last year - and doesn't like to be away from him.
A source said: "Jen is totally into him. They have been hanging out for months.
"She wants to be with him all the time. She's diving in headfirst."