Eugenie, 28, is ninth in line to the British throne.
On her head, she wore one of the Queen's personal tiaras, the Greville Emerald Kokoshnik, and no veil.
Eugenie and Jack Brooksbank are to marry at St. George's Chapel, which is part of the castle complex, on a gusty day that required early arrivals to hold onto their elegant hats as they crossed the manicured grounds.
Beatrice is wearing custom Ralph & Russo couture and accessories.
Truly, it was a wedding fit for a princess.
Amid gushing compliments about her looks, her wedding dress and the beauty of the ceremony, commentators also unleashed their anger at the cost of yet another royal wedding in Britain this year.
Duchess of Cambridge thrills in off-the-shoulder £1395 Erdem dress
Last month Prince William opened up about the emotional toll of witnessing suffering during his time as an air ambulance pilot. He added: "She was genuinely knowledgeable and excited about photography - particularly the history".
Stars from the world of showbusiness and sport attended Harry's wedding and Buckingham Palace announced that one of Eugenie's bridesmaids will be Theodora Williams, the six-year-old daughter of pop star Robbie Williams and his wife Ayda Field. They married in the same venue used by Harry and Meghan in May.
Several of the Queen's great-grandchildren - including Prince George and Princess Charlotte - will be in the bridal party, while Eugenie's sister, Princess Beatrice, will be the blushing bride's maid of honour.
Eugenie's elder sister Princess Beatrice, the maid of honor, read a passage from the "The Great Gatsby" describing the smile of the title character, a rich man famed for throwing parties for the 1920s NY glitterati. She read a selection from F. Scott Fitzgerald's "The Great Gatsby" during the service.
London-based cake designer Sophie Cabot, 33, was chosen to make the cake for Eugenie, who is the younger daughter of Queen Elizabeth's second son, Prince Andrew the Duke of York, and his former wife, flame-haired Sarah Ferguson.
SECURITY BILL After the service, the couple will embark on an open-top carriage tour of Windsor.
Prior to the big day, Eugenie spoke with ITV about her decision.
The couple invited 1,200 members of the public to come onto the castle grounds for a closer glimpse of proceedings. There were also crowds of well-wishers on the streets outside the imposing castle.