Kate, 37, chose a sunny yellow v-neck dress with a fitted waistband by Sarah Burton of Alexander McQueen, with a matching floral hat by Philip Treacy.
The mum-of-three completed her outfit with the Queen’s Bahrain Pearl Drop Earrings, which were a wedding gift to the then-Princess Elizabeth in 1947.
Kate has previously worn the jewels - for the Remembrance Sunday service in 2016 and for a church service in Balmoral last summer.
The duchess has attended Trooping the Colour every year since her marriage to Prince William in 2011. This is seventh time she’s chosen an outfit by Alexander McQueen for the occasion.
Kate shared a carriage with the Duchess of Cornwall and the Duke and Duchess of Sussex - who made her first public appearance since the birth of son Archie last month.
Meghan, 37, wore a navy ensemble by her wedding dress designer Clare Waight Keller of Givenchy, while Camilla, 71, chose a green Bruce Oldfield embroidered dress.
Following the parade, the royals joined the Queen on the Buckingham Palace balcony for the RAF fly-past, where the Duke and Duchess of Cambridge’s youngest child Prince Louis made his debut.
Earlier this week, the Cambridges attended the state banquet for US president Donald Trump and his wife Melania.
Kate wore a white ruffled gown by Alexander McQueen with Princess Diana’s Lover’s Knot tiara and the Queen Mother’s sapphire and diamond fringe earrings.
She then made her debut at the Beating Retreat military music concert on Thursday night, where she took the salute.
Next week, the Cambridges will resume their royal duties with a trip to Cumbria on Tuesday.