The royals watched swimming events at the Sandwell Aquatics Centre in Birmingham, along with Prince William's uncle Edward, Earl of Wessex, vice-patron of the Commonwealth Games Federation, and his wife Sophie, Countess of Wessex, and children, James, Viscount Severn and Lady Louise Windsor.
For the sporting day out, Duchess Kate wore a white Alexander McQueen wool suit with a white T-shirt and accessorised with a white belt, Camilla Elphick taupe and black slingback shoes, and a Mappin & Webb diamond earring and necklace set.
She wore her hair long, in loose waves, and looked animated as she watched the in-pool action.
Princess Charlotte wore her hair in pigtail plaits and sported a black-and-white striped summer dress by British children's brand Rachel Riley. Prince William wore a blue shirt and relaxed blazer for the appearance.
It's been a big sporting week for Princess Charlotte, who shared a message for the England Women's Euro 2022 team ahead of their final against Germany on Sunday. The Lionesses went on to beat Germany 2-1 in extra time.
"We both want to wish the Lionesses the best of luck for tonight. You’ve done amazingly well in the competition and we are rooting for you all the way," Prince William said in a message shared on social media.
The video was filmed in a garden and Princess Charlotte said: "Good luck, I hope you win."
Prince William, who is president of The Football Association, attended the game and presented the team with their medals. He also congratulated the team, tweeting: “An incredible win @lionesses and the whole nation couldn’t be prouder of you all.
“Wonderful to see history in the making tonight at Wembley, congratulations! W.”