Princess Eugenie is marrying her long-term partner Jack Brooksbank at St George's Chapel in Windsor.
The Royal Family and celebrities are among the 850 guests at the ceremony.
The ninth in line to the throne was given away by her father, Prince Andrew - watched by her grandmother, the Queen.
Prince Philip, Eugenie's mother, the Duchess of York, and her sister Beatrice, are also attending, along with crowds of well-wishers outside.
Around 1,200 members of the public selected by ballot are listening to a live broadcast in the castle grounds, and thousands are watching outside in extremely windy conditions.
The wedding is being screened in full on ITV as part of a three-hour special of the daily magazine show, This Morning.
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The 28-year-old and drinks executive Jack Brooksbank, 32, are tying the knot at St George's Chapel - where Prince Harry and Meghan Markle married in May.
The couple met when skiing in Switzerland in 2010.
The Duke and Duchess of Cambridge's children, Prince George and Princess Charlotte, are taking on pageboy and bridesmaid roles.
The other bridesmaids are Mia Tindall - daughter of Zara and Mike Tindall - Mia's cousins Savannah and Isla, Eugenie's goddaughter Maud Windsor and Robbie Williams' daughter, Theodora.
The other pageboy is Louis de Givenchy, six, the son of friends.
Eugenie's sister, Princess Beatrice, is maid of honour. Mr Brooksbank has chosen his brother, Thomas, as best man.
Among the celebrities sharing their special day are model Cara Delevingne, singer Robbie Williams, comedian Jimmy Carr and David Emanuel - who designed Princess Diana's wedding dress.
Blustery Windsor waits for crowds
By Jonny Dymond, BBC royal correspondent
It's cool and blustery at Windsor and outside the castle the crowds are pretty thin.
It's a far cry from the last royal wedding here when, by now, the pavements were jammed with people.
But there's still some time until the carriage ride, so people may be looking at the overcast skies and hedging their bets.
Inside St George's Chapel, autumnal flowers line the stalls of the Quire leading up to the altar.
Just one ring will be exchanged in today's ceremony - Jack Brooksbank will join Prince William in keeping his hand unadorned.
Fans of the Royal Family are joining in the spirit from outside the castle walls.
Karen Levet and Kim Russell, wearing matching flower crowns in red, white and blue, travelled from Cambridge.
"We missed Harry's wedding so we thought we needed to be at this one," said Ms Levet. "There's a really happy atmosphere here."
Her friend added: "I know people must think we're mad but we love the Royal Family and wanted to be here for the big day, so we're happy to be here early!"
Italian tenor Andrea Bocelli is performing two pieces during the ceremony, which will also feature music from members of the Royal Philharmonic Orchestra.
The Archbishop of York, Dr John Sentamu, has also prepared a personal prayer.
Dean of Windsor David Conner is officiating and will give an address.
The couple will travel from St George's Chapel along Windsor High Street and the Long Walk, before an afternoon reception hosted by the Queen in St George's Hall at the castle.
An evening party will take place at the York family home, the Royal Lodge, in Windsor Great Park.
The families are paying for the private aspects of the wedding.
However, republican campaigners say taxpayers face an estimated £2m security bill.
The campaign group Republic argues that as Princess Eugenie, the ninth in line to the throne, does not carry out public duties, the families should foot the bill.
Buckingham Palace says this is a matter for the police.