Kate, 44, led the royal family in the traditional carriage procession as she arrived at the Berkshire racecourse, appearing in bright spirits as she waved to crowds.
The Princess looked radiant in bright yellow, marking her third public appearance in a week after attending Saturday's Trooping the Colour in London and Monday's Garter Day service in Windsor.
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The appearance also marks the end of her three-year hiatus from the event, and one year since the royal mother-of-three unexpectedly pulled out of Ascot at the last minute in 2025.
It was understood that her absence last year was part of finding the right balance in returning to public duties following her battle with cancer, for which she is now in remission.
Meanwhile, Prince William, who celebrates his 44th birthday on Sunday, looked as handsome as ever in a suit, matching his wife perfectly with flowers in his buttonhole.
The Prince and Princess of Wales were in the second carriage, alongside the Duke and Duchess of Richmond and Gordon.
Politician and broadcaster Rory Stewart was in the third carriage with his wife, Shoshana Clark, alongside Queen Camilla's companions, Lady Sarah Keswick and Fiona Petty-Fitzmaurice, Marchioness of Lansdowne.
The royal party were cheered as they travelled past the stands filled with racegoers waiting for the first race of the day.
The Princess's attendance was confirmed by organisers with just two hours to go, while Prince William is expected to present the trophy to the winner of the Prince of Wales's Stakes later in the afternoon – one of the event's most prestigious titles.
When the carriages reached the parade ring, where the jockeys mount their rides and winners are welcomed, the Waleses stepped down from their coach and when the King greeted them Kate curtseyed to her father-in-law.
She chatted with the Queen while William spoke to other guests.
HELLO!'s resident racing expert, Charlie Atkin, shared his verdict on the royal runners ahead of day two of Royal Ascot.
"While yesterday's royal runner might have disappointed, the King and Queen have another live chance on day two," he states. "Point Of Law runs in the Queen's Vase (3.05pm) for Charles and Camilla, a race previously won by the late Queen with Estimate in 2012.
"Point Of Law has only run twice but finished second and first in those two starts. He currently sits around 8/1 in the betting and has a great each way chance if appreciating the step up in distance."
On Monday, King Charles and Queen Camilla attended the opening day of Royal Ascot, but their racing dreams were soon dashed when their horse was beaten on the track.
They were joined by the Princess Royal and her son Peter Phillips, alongside his new bride, Harriet. Other guests included the Queen’s son, Tom Parker Bowles.
All the unforgettable moments from the King's Birthday Parade - from the dazzling carriage procession and warm family interactions to the spectacular RAF flypast