Middleton covered her burgeoning baby bump in a blue velvet dress by London-based designer Erdem. The elegant frock featured a white floral print, bell sleeves and a solid hemline. She paired the gown with black Gianvito Rossi "D’Orsay" suede sandals and minimal jewelry, including silver earrings and her wedding ring.
Middleton's brown locks were pulled back in a neat updo and her makeup consisted of blush, smoky brown eyeshadow and a light pink lip.
As for William, he matched his lovely wife by donning a bright royal blue suit with a crisp white button-up and a printed blue tie.
The Duke and Duchess were also accompanied by Crown Princess Victoria and Prince Daniel as they greeted guests inside the event.
During the glam evening, William took the stage to talk about his and his wife's time in Sweden.
"In the past two days, Catherine and I have experienced some of the unique mix of factors that contribute to the Swedish way of life," he expressed. "The Swedish love of the outdoors -- the way you embrace your climate and environment and are committed to ensuring future generations can do the same. The fact that you do so when it is so cold is really inspiring. One lesson that we will take home with us is that children are actively encouraged to spend time outdoors, whatever the weather."
The Duke said: "Tonight is a celebration of energy, creativity, innovation and engagement in the world - all words we associate with both Britain and Sweden. Above all, tonight is a celebration of friendship. Skål!" pic.twitter.com/37wWJ19QEK
Earlier that day, the couple -- who are expecting their third child together -- visited the Karolinska Institute to learn more about Sweden's pioneering work in the field of mental health and children's well-being, and how children and young teenagers are supported through the Swedish education system.
On Thursday, the royal couple will be meeting representatives of Sweden's armed forces before heading off to Oslo, where they will be greeted by Crown Prince Haakon and Crown Princess Mette-Marit of Norway.
For more on Middleton's recent fashion, watch below.