Léon Joseph Florentin Bonnat (June 20, 1833 – September 8, 1922) was a French painter.

He was born in Bayonne, but from 1846 to 1853 he lived in Madrid, Spain, where his father owned a bookshop. While tending his father's shop, he copied engravings of works by the Old masters, developing a passion for drawing. In Madrid he received his artistic training under Madrazo. He later worked in Paris, where he became known as a leading portraitist, never without a commission. His many portraits show the influence of Velázquez, Jusepe de Ribera and other Spanish masters, as well as Titian and Van Dyke, whose works he studied in the Prado. Following the period in Spain Bonnat worked the ateliers of the history painters Paul Delaroche and Leon Cogniet (1854) in Paris. Despite repeated attempts, he failed to win the prix de Rome, finally receiving only a second prize. However, a scholarship from his native Bayonne allowed him to spend three years in Rome (1858 – 60) independently. During his stay in Rome, he became friends with Edgar Degas, Gustave Moreau, Jean-Jacques Henner and the sculptor Henri Chapu.

He won a medal of honor in Paris in 1869, going on to become one of the leading artists of his day. Bonnat went on to win the Grand Officer of the Légion d'honneur and became a professor at the Ecole des Beaux Arts in 1882. Bonnat was quite popular with American students in Paris. In addition to his native French, he spoke Spanish and Italian and knew English well, to the relief of many monolingual Americans. In May 1905 he succeeded Paul Dubois as director of the Ecole des Beaux-Arts. Bonnat "was a liberal teacher who stressed simplicity in art above high academic finish, as well as overall effect rather than detail," explains Julius Kaplan. Bonnat's emphasis on overall effect on the one hand, and rigorous drawing on the other, put him in a middle position with respect to the Impressionists and academic painters like his friend Jean-Léon Gérôme.

Bonnat's vivid portrait-painting of contemporary celebrities is his most characteristic work, but his most important works are arguably his powerful religious paintings, such as his Christ on the Cross (now in the collection of the Musée du Petit Palais in Paris, but not currently on display), Job (in the Musée Bonnat), St Vincent Taking the Place of Two Galley Slaves (at the church of Saint-Nicholas des Champs in Paris), and the large Martyrdom of St Denis for the Pantheon in Paris. However, he received few commissions for religious and historical paintings, and most of his output consists of portraits. He also produced genre paintings of Italian peasants, and a small number of Orientalist scenes.

The writers Émile Zola and Théophile Gautier were among Bonnat's supporters. Gautier hailed him as "the antithesis of Bouguereau," because of the stark naturalism and lack of surface finish that characterize Bonnat's work. Bonnat is often wrongly classed as an academic painter. In fact, he had friends and connections among the independent artists of his time, such as Edgar Degas, whom he met during his stay in Rome and who painted two portraits of Bonnat, and Édouard Manet, who shared his predilection for Spanish painting. He taught together with Pierre Puvis de Chavannes in the private atelier he ran before becoming professor at the École. He supported Auguste Rodin's candidacy for the Institut, and defended Gustave Courbet's submissions to the salon.

In a gesture of gratitude for the help he had been provided in his youth, Bonnat built a museum in his native city of Bayonne, the Musée Bonnat. Most of the works in the museum are from Bonnat's personal collection of works of art, amassed over a lifetime of travelling around Europe. It includes an exceptionally fine collection of old master drawings from Leonardo da Vinci and Michelangelo to Ingres and Géricault. The artist died in 1922.

Some of Bonnat's more notable students include: John Singer Sargent, Stanhope Forbes, Gustave Caillebotte, Prince Eugen, Duke of Närke, Gustaf Cederström, Laurits Tuxen, P.S. Krøyer, Suzor-Coté, Alfred Philippe Roll, Georges Braque, Thomas Eakins, Raoul Dufy, Jean Béraud, Marius Vasselon, Hubert-Denis Etcheverry, Fred Barnard, Louis Béroud, Paul de la Boulaye, Aloysius O'Kelly, Erik Werenskiold, Edvard Munch, Alphonse Osbert, Henry Siddons Mowbray, Charles Sprague Pearce, Henri de Toulouse-Lautrec and Walter Tyndale.