The duchess accompanied her children, Prince George and Princess Charlotte.
Kate also caused controversy when it was caught on camera that she and Camilla, the Duchess of Cornwall, were snickering at the priest's speech during the ceremony.
Kate Middleton has made her first official public appearance since welcoming her third child, Prince Louis, on April 23.
Stepping out for Prince Harry and Meghan Markle's wedding, Kate looked stunning in cream as she made her way into St George's Chapel at Windsor Castle.
The duchess accompanied her children, Prince George and Princess Charlotte.
Kate also caused controversy when it was caught on camera that she and Camilla, the Duchess of Cornwall, were snickering at the priest's speech during the ceremony.