The 28-year-old American actress and 39-year-old Canadian actor attended the event Thursday to honor Canadian Prime Minister Justin Trudeau and his wife, Sophie.
Lively and Reynolds looked the picture perfect couple, with the actress in a silk Ralph & Russo gown and Reynolds in a navy Burberry tuxedo. Lively completed her old Hollywood look with dazzling Lorraine Schwartz jewelry.
"Sophie and I could not have asked for a better evening. Thank you to the President & First Lady for your warmth, friendship & hospitality," he added.
First ladies Sophie Grégoire Trudeau and Michelle Obama also brought the glam by both wearing Canadian designers. The Canadian first lady wore a Lucian Matis dress with a train, while Obama chose a floral Jason Wu design.
Singer Sara Bareilles, Saturday Night Live executive producer Lorne Michaels and NHL commissioner Gary Bettman were among the other guests at the event. The Washington Post described the dinner as "a cozy cross-border affair."
Lively and Reynolds married in 2012 and welcomed their first child, daughter James, in 2014. The actress will star in All I See is You with Jason Clarke, while Reynolds is slated for Criminal with Gal Gadot and The Hitman's Bodyguard with Elodie Yung.