Her Royal Highness The Princess Louise, Princess Royal and Duchess of Fife (Louise Victoria Alexandra Dagmar Duff, née Wettin) (20 February 1867-4 January 1931), was the third child and the eldest daughter of King Edward VII and Queen Alexandra. She was the younger sister of King George V and the fifth daughter of a British monarch to be styled Princess Royal.

Princess Louise of Wales was born at Marlborough House, the London residence of her parents, the Prince and Princess of Wales (later King Edward VII and Queen Alexandra). She spent much of her childhood at Sandringham House, her parents' country estate in Norfolk. Like her sisters, Princess Maud and Princess Victoria, she received a limited formal education.

On 27 June 1889, Princess Louise married Alexander Duff, 6th Earl Fife (11 October 1840-29 December 1912), at the Private Chapel in Buckingham Palace. Two days after the wedding, Queen Victoria created him Duke of Fife and Marquess of Macduff in the Peerage of the United Kingdom. The Letters Patent creating this dukedom contained the standard remainder to "male heirs of the body lawfully begotten." However, it eventually became apparent that the Duke and Duchess of Fife would not have a son. Therefore, on 24 April 1900, Queen Victoria signed Letters Patent creating a second Dukedom of Fife, along with the Earldom of Macduff in the Peerage of the United Kingdom with a special remainder: in default of a male heir, these peerages would pass to the daughters of the 1st Duke and then to their male descendants.

The Duke of Fife and Princess Louise, Duchess of Fife had three children:

• Alaistar Duff, Marquess of Macduff (stillborn 1890).
• Lady Alexandra Duff (17 May 1891-26 February 1929) m. her cousin Prince Arthur of Connaught (13 January 1883-12 September 1938), and had issue.
• Lady Maud Duff (3 April 1893-14 December 1945) m. Charles Carnegie, 11th Earl of Southesk, and had issue.

On 9 November 1905, King Edward VII declared Princess Louise the Princess Royal, the highest honor bestowed on a female member of the royal family. Therefafter, she was styled HRH The Princess Royal. At the same time, the King declared that the two daughters of the Princess Royal would have the titular dignity of Princess of Great Britain and Ireland and the style of Highness, with precedence immediately after all members of the British Royal Family styled Royal Highness. From that point ownward, the Princess Royal's daughters, Princess Alexandra of Fife and Princess Maud of Fife, no longer took their rank from their father, but rather from the will of the Sovereign.

In December 1911, while sailing to Egypt, the Princess Royal and her family were shipwrecked off the coast of Morocco. Although they escaped unharmed, the Duke of Fife fell ill with pleurisy, probably contracted as a result of the shipwreck. He died at Assuan, Egypt in January 1912, and Princess Alexandra succeeded to his Dukedom, becoming Duchess of Fife in her own right.

Princess Louise of Wales received the Order of Victoria and Albert in 1885 and the Imperial Order of the Crown of India in 1887. She became a Lady of the Venerable Order of St. John of Jerusalem (LJStJ) in 1888. In 1929, the Princess Royal became a Lady Grand Gross of the Order of St. John of Jerusalem (GCStJ). She became colonel-in-chief of the 7th Dragoon Guards (the Princess Royal's Own) in 1914. She later served as colonel-in-chief of the 4th and 7th Dragoon Guards when it was formed in 1921.

The Princess Royal died in January 1931, at her home in Portman Square, London and was buried in St. George's Chapel, Windsor Castle. Her remains were later removed to the Private Chapel, Mar Lodge Mausoleum, Braemar, Aberdeenshire.