Gaston Louis Alfred Leroux was a French journalist and author of detective fiction.

In the English-speaking world, he is best known for writing the novel The Phantom of the Opera (Le Fantôme de l'Opéra, 1910), which has been made into several films and stage productions of the same name, such as the 1925 film starring Lon Chaney, and Andrew Lloyd Webber's 1986 musical. It was also the basis of the 1990 novel Phantom by Susan Kay. His 1907 novel The Mystery of the Yellow Room is one of the most celebrated locked room mysteries.

Leroux went to school in Normandy and studied law in Paris, graduating in 1889. He inherited millions of francs and lived wildly until he nearly reached bankruptcy. Then in 1890, he began working as a court reporter and theater critic for L'Écho de Paris. His most important journalism came when he began working as an international correspondent for the Paris newspaper Le Matin. In 1905 he was present at and covered the Russian Revolution. Another case he was present at involved the investigation and deep coverage of an opera house in Paris, later to become a ballet house. The basement consisted of a cell that held prisoners in the Paris Commune.

He left journalism in 1907 and began writing fiction. In 1919, he and Arthur Bernède formed their own film company, Société des Cinéromans, publishing novels and turning them into films. He first wrote a mystery novel titled Le mystère de la chambre jaune (1908; English title: The Mystery of the Yellow Room), starring the amateur detective Joseph Rouletabille. Leroux's contribution to French detective fiction is considered a parallel to those of Sir Arthur Conan Doyle in the United Kingdom and Edgar Allan Poe in the United States.

Leroux was made a Chevalier de la Legion d'honneur in 1909. He died at age 58 in Nice, France in 1927.