The 35-year-old Duchess wore a stunning beaded Jenny Packham dress to the annual event.
The long-sleeve dress featured elegant beading and a sheer overlay, nipped in at the waist to give a cover to her blossoming baby bump.
Meanwhile, Prince William walked proudly beside her dressed in a smart black tuxedo.
The couple sat down to watch acts such as Louis Tomlinson, Paloma Faith, The Killers, The Script and Seal at the gala event at the London Palladium.
The couple last attended the event in 2014 when Kate, who was then pregnant with Princess Charlotte, showed off her bump in a black lace gown from Diane von Furstenberg - a dress also owned by Prince Harry's girlfriend and rumoured fiancée, Meghan Markle.