The 32-year-old royal looked on happily as Adam wed interior designer Rebecca Poynton at London's luxury Dorchester Hotel.
Kate's parents Carole and Michael Middleton were also present, along with her siblings James and Pippa – who attended with her boyfriend Nico Jackson.
The groom Adam is the son of Michael's brother, Richard. He works for Mayfair firm Manchester Square Partners and advises senior business executives.
The wedding, which took place a day before the bride's 30th birthday, is said to have included a civil ceremony, and was held in the hotel's penthouse and pavilion.
According to royal photographer Niraj Tanna, who was tasked with capturing the happy couple's big day on camera, Kate entered the building via the underground car park.
She was reportedly accompanied by nine protection officers while other family members arrived separately.
The bride and groom's choice of venue for the happy occasion comes at an opportune time for the luxury hotel.
Its owner, the Sultan of Brunei, has recently imposed a series of harsh Islamic anti-gay laws, sparking a worldwide boycott of the Dorchester Collection hotels.
Celebrities such as Stephen Fry, Sir Richard Branson, Vogue editor Anna Wintour and TV presenter Ellen DeGeneres have all backed the boycott in response to the extreme laws.
Other properties owned by the Dorchester Collection include the Beverly Hills Hotel in Los Angeles, Le Meurice in Paris and Coworth Park in Ascot, where Prince William and Harry took part in the Audi Polo Challenge at the weekend.