The "Hitch" beauty, 40, is said to be seven months pregnant.
The couple began dating in 2011 after shooting crime drama "The Place Beyond the Pines" together.
Representatives for the actors had yet to respond to requests for a comment as WENN went to press.
The baby news comes five months after Mendes, 40, denied rumors she was expecting after asking to undergo a manual body search at an airport security checkpoint instead of walking through the X-ray scanners.
She laughed off the speculation at the time, telling talk show host Ellen DeGeneres, "It's (the baby rumors are) so ridiculous. It all started because I didn't want to go through the scanners at the airport. You know those X-ray scanners, which are really creepy? They basically see you naked. And not only that, but there's a radiation aspect to it. So I always opt out. I always ask for a personal pat-down. ... (Tabloid reporters thought) that was good enough reason to say 'Hey (she's pregnant).'"
When DeGeneres pressed her further about whether there was any truth to the pregnancy rumor, Mendes replied, "Oh God! No, no no!"
Mendes previously dated filmmaker George Augusto, while Gosling, 33, enjoyed romances with his "The Notebook" co-star Rachel McAdams and Sandra Bullock.