Kate wore the Lover's Knot tiara, which was beloved by Princess Diana. The tiara was created by Queen Elizabeth's grandmother, Queen Mary, in 1913.
She completed her glamorous look with a white Alexander McQueen gown -- the luxury fashion house that designed the wedding dress she wore back in 2011 -- and diamond and sapphire earrings she borrowed from Queen Elizabeth II herself.
Camilla, the Duchess of Cornwall, also attended the glitzy affair, donning a cream-colored gown by Bruce Oldfield.
The queen wore a white gown by her personal dressmaker Angela Kelly that featured crystal centered daisies, and paired it with a burmese ruby and diamond tiara.
Although the president was visiting, the American Duchess Meghan was absent from the event as she is on maternity leave after giving birth to her first child last month.
SEE MORE: President Donald Trump, First Lady Melania, meet with Queen Elizabeth at Buckingham Palace