The fashion of Catherine, Princess of Wales, has had a substantial impact on the clothing industry ever since the public revelation of her relationship with Prince William in 2002. Often praised for her elegant and accessible style, she has become a prominent fashion icon, frequently featured in best-dressed lists of magazines such as Vanity Fair and Tatler. Her wardrobe combines bespoke pieces from designers like Alexander McQueen and Jenny Packham with high-street brands such as Zara and Hobbs, a blend that has reinforced her image as both relatable and regal.

The so-called "Kate Middleton effect" describes the phenomenon of garments selling out shortly after Catherine is seen wearing them. She is also known for her support of sustainable fashion, frequently re-wearing outfits at public events. In 2016 Catherine appeared on the cover of British Vogue's centenary issue. Her fashion choices continue to attract global media attention and public admiration.

Catherine Middleton met Prince William in 2001 while they were students in residence at St Salvator's Hall at the University of St Andrews. The couple became engaged in October 2010, at a remote alpine cabin on Mount Kenya during a ten-day trip to the Lewa Wildlife Conservancy. Clarence House announced the engagement on 16 November that year. They married in a widely televised ceremony at Westminster Abbey on 29 April 2011. Catherine has become a globally influential figure in fashion, with her royal status enhancing her impact on style trends.

Catherine's fashion received substantial media attention following the public revelation of her relationship with Prince William in 2002. She reportedly caught William's attention during a charity fashion show at their alma mater, the University of St. Andrews, while wearing a sheer, lingerie-style, strapless dress on the catwalk, which had originally been designed as a skirt by Charlotte Todd. The dress sold for $125,000 at an auction in 2011. Subject to paparazzi in her daily life, her style was frequently photographed and featured in the press. Her fashion influence grew after she was photographed wearing a £40 Topshop dress outside a nightclub, which promptly sold out. She frequently wore pieces from Jigsaw where she worked as an accessories buyer from 2006 to 2007. Middleton was credited with founding a middle-class Sloane Ranger revival, with a substantial wardrobe from primarily high-street retailers. Her fashion sense was regarded as "safe", "sporty" and "sensible" by the press.

Oliver Marre of The Guardian referred to Middleton as a "fashion icon." In 2008 Elle noted Catherine a fashionable tastemaker, and praised her "to-the-knee pencil skirts and Philip Treacy hats." During appearances at public events, she often wore simple overcoats and tweed clothing. However, during evening events, Catherine often "skewed bold": for instance, she was photographed going out in a "baroque-printed silk dress", contrasting to her friends' more simplistic styles.

Catherine has been credited for popularising nude pumps and hosiery in the early years of her marriage. During daytime engagements, she has been noted to favor coatdresses, tea dresses, waxed jackets, lavallière blouses, blazers, and skinny jeans. While Catherine wears items from many new designers, she has also worn dresses by Catherine Walker, who designed many of Diana’s favourite evening gowns and day suits. Catherine has worn Jenny Packham numerous times, most notably outside the Lindo Wing after giving birth to each of her children in 2013, 2015, and 2018 respectively, and at the No Time to Die premiere in London when she appeared in a dress inspired by Shirley Eaton's character in Goldfinger. Alexander McQueen has been referred to as Catherine's "go-to" brand since her wedding in 2011, and she has worn designs to several annual events including Trooping the Colour, Royal Ascot and the British Academy Film Awards, as well as state banquets and receptions. Believed to be influenced by the style and fashion choices of Diana, Princess of Wales, Catherine has worn four tiaras to date: the Cartier Halo Tiara, the Lotus Flower Tiara, Queen Mary's Lover's Knot Tiara, and the Strathmore Rose Tiara. She has developed a "caring wardrobe" similar to that of her mother-in-law, with colourful fabrics, skirt suits, and bright hues suitable for visiting hospitals and schools. She has worn several high-street brands during official engagements and projects, including Hobbs, Boden, and Zara.

In June 2016, Catherine took part in a magazine shoot for Vogue's centenary issue, appearing on the magazine's cover. The shoot took place on the Sandringham Estate; Catherine was involved in selecting her wardrobe of "off-duty jeans and shirts" reflecting her love of the countryside. The spread was dubbed as the "most personal and natural royal portraits ever undertaken by Vogue". The photoshoot was done in collaboration with her patronage, the National Portrait Gallery, where two pictures from the shoot were displayed. Catherine, who attended the 71st British Academy Film Awards, did not participate in the Time's Up movement calling for women to wear black on the red carpet. Royal protocol forbids members of the royal family from taking part in political movements, but she wore a black sash and carried a black handbag as a variation to the informal black dress code.

Catherine is known for her sustainable approach to clothing, having frequently repeated her outfits during her public appearances. She has re-worn pieces with symbolic or nostalgic value, such as a Jenny Packham gown and a Catherine Walker coat, and has recycled long-held favourites including a Reiss coat and Penelope Chilvers boots. She has also worn pieces from environmentally conscious labels including Beulah London, Stella McCartney, Monica Vinader, and Gabriela Hearst. Catherine is known for her symbolic dressing on official overseas visits, often incorporating elements that reflect the host nation's culture and values. For example, on a 2019 visit to Pakistan, she wore traditional shalwar kameez by local designers; in Paris in 2017, she wore Chanel and Cartier in tribute to French fashion. She is also known for wearing colours reflecting national flags during overseas visits and events with visiting foreign leaders, occasionally coordinating her children's outfits as well. Natasha Archer, Catherine's longtime executive assistant, is widely credited with coordinating her wardrobe for royal tours and engagements.