The Duchess of Edinburgh's Trooping the Colour dress retails for a whopping £750.
Sophie, 61, travelled in the No.1 Barouche carriage alongside Sir Tim Laurence, Princess Anne's husband, as they celebrated King Charles's official birthday.
The duchess opted to wear a short-sleeved midi dress with a collar from Roland Mouret, a brand loved by the Princess of Wales.
Sophie accessorised with a Jane Taylor feather-trimmed saucer hat that she has been wearing since 2017.
The straw saucer hat trimmed with feathers is unfortunately no longer available to purchase.
The mother-of-two could be seen wearing modern diamond and pearl drop earrings, as well as a diamond rose brooch.
Sophie's husband, Prince Edward, is wearing the Full Ceremonial Guard Order of the Scots Guards.
His uniform bore the cypher of King Charles as a Personal Aide-de-Camp to His Majesty and the cypher of Queen Elizabeth II, as Edward was Personal Aide-de-Camp to his late mother.
Sophie rode in a carriage alongside Sir Tim Laurence, Princess Anne's husband
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Also attending today was the Princess of Wales, who wore her "Cavolfiore" earrings with 18-carat yellow gold from Cassandra Goad.
The cluster earrings, which retail for £7,245, were famously worn at Prince Louis' christening in 2018.
Catherine paired her earrings with a bespoke light blue "Lafayette" coat dress from Catherine Walker, a beloved brand of the late Princess Diana's.
In addition, she wore a Philip Treacy hat and an Irish Guards brooch as Catherine is Colonel of the Irish Guards.
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The Duchess of Edinburgh's hat was first worn in 2017
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Trooping the Colour, commonly known as "The King’s Birthday Parade", is a military ceremony dating back to the early eighteenth century.
The colours (flags) of the battalion are carried (or "trooped") down the ranks to teach soldiers to recognise them.
Catherine, 44, travelled in the Ascot Landau carriage with her children, Prince George, Princess Charlotte and Prince Louis, as they made their way down The Mall to Buckingham Palace.
The Ascot Landau is a four-wheeled luxury carriage with a roof that can be let down.
The Duchess of Edinburgh wore diamond and pearl jewellery for the occasion
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Since 1748, this parade has marked the Sovereign's Official Birthday, despite King Charles being born on November 14, 1948.
This year marks the fourth Trooping the Colour of King Charles’s reign.
The King attended his first Trooping in 1951, aged three, riding in a carriage with his grandmother, the Queen Mother, and aunt, Princess Margaret.