The mother-of-one, 54, was a vision, wearing an elegant duck egg blue gown by Suzannah London, complete with a matching hat by Jane Taylor London. As for accessories, the NHS nurse opted for Jimmy Choo stilettos, a bespoke Anya Hindmarch handbag, and was decked out in jewels by Pragnell.
As for her new husband, Peter Phillips looked incredibly smart in his ensemble, complementing his wife's outfit, opting for a tie in a hue of the same blue colour palette.
Peter - who shares two daughters, Savannah and Isla, with his ex-wife Autumn Phillips - was beaming beside his wife, the newlyweds joining their royal family members, King Charles, Queen Camilla, Zara and Mike Tindall at the annual horse event.
Harriet's sister-in-law Zara looked beautiful for the occasion; the equestrian, 45, rocked a bubblegum puff-sleeve midi dress with a belted waist and scalloped lapels, accessorising with a matching hat, suede heels and a clutch bag.
Ascot is an occasion beloved by many members of the British royal family, but in particular for the King and Queen, who are racehorse owners.
HELLO!'s resident racing expert Charlie Atkin believes the King and Queen have a golden opportunity to add another Royal Ascot success to their record.
He states: "King Charles and Queen Camilla will be in attendance today not just as King and Queen, but also as racehorse owners. While they have a few runners across the week, their best chance of a winner will be in the Ascot Stakes, going off at 5 pm today.
Peter Phillips and Harriet Sperling will be marrying in a Cotswolds ceremony on Saturday, and on Friday they were seen with other royals at a rehearsal dinner