The pregnant royal wore her late mother-in-law, Princess Diana's favorite tiara to the prestigious event.
And she looked smashing it in!
The Queen Mary's Lover's Knot tiara was placed into her shiny brown hair that was pulled back into a beautiful updo.
She complimented it with a diamond necklace and pearl and diamond drop earrings.
The tiara, which Queen Mary made around 1914, is more than 100 years old. Middleton also wore it to the annual event last year.
What do you think of the tiara? Share with us in the comments.