The 36-year-old actor and 43-year-old actress made a rare public appearance as a couple at a Saturday Night Live after-party at TAO nightclub in Manhattan, according to People.
Gosling, who wore a denim jacket, patterned sweater and blue pants, was spotted holding hands with Mendes as they arrived at the venue. The actress stunned in a belted jumpsuit with a white, pink and black print.
E! News reported Jay-Z and Beyoncé were among the other celebrities at the event. The party also marked a first public appearance for Scarlett Johansson and her boyfriend, SNL Weekend Update co-anchor Colin Jost.
"[Scarlett and Colin are] going strong," a source said.
Gosling had hosted SNL for his second time earlier in the night. He gave a shoutout to Mendes and their two daughters, 3-year-old Esmeralda Amada and 17-month-old Amada Lee, during the episode.
"Eva, Ese, Amada, I love you!" the actor said.
Gosling and Mendes started dating in 2011 after meeting on the set of The Place Beyond the Pines. The actress said in the April issue of Shape that she loves "being home" with family and friends instead of on the red carpet.