She was first spotted wearing the bling on July 25 while promoting Estee Lauder's new Dimensions Skincare line at Nordstrom's Aventura Mall in Florida, reported Aceshowbiz.
Mendes, 41, has been flashing the alleged engagement ring in photos she shared on her Instagram.
Neither the actress nor Gosling, 34, has addressed the rumours. Her representative also has not yet made any comment on the matter.
Mendes and Gosling first met in the fall of 2011 on the set of their drama flick "The Place Beyond the Pines".