Wilhelm Albert Włodzimierz Apolinary Kostrowicki, known as Guillaume Apollinaire (Rome, 26 August 1880 – 9 November 1918, Paris) was a French poet, playwright, and art critic born in Italy to a Polish mother.

Among the foremost poets of the early 20th century, he is credited with coining the word Surrealism and writing one of the earliest works described as surrealist, the play The Breasts of Tiresias (1917, used as the basis for a 1947 opera). Two years after being wounded in World War I, he died in the Spanish flu pandemic of 1918 at age 38.

Born Wilhelm Apollinaris de Kostrowitzky and raised speaking French, among other languages, he emigrated to France and adopted the name Guillaume Apollinaire. His mother, born Angelica Kostrowicka, was a Polish noblewoman born near Navahrudak (now in Belarus). Apollinaire's father is unknown but may have been Francesco Flugi d'Aspermont, a Swiss Italian aristocrat who disappeared early from Apollinaire's life. Apollinaire was partly educated in Monaco.

Apollinaire was one of the most popular members of the artistic community of Montparnasse in Paris. His friends and collaborators in that period included Pablo Picasso, Gertrude Stein, Max Jacob, André Salmon, Marie Laurencin, André Breton, André Derain, Faik Konica, Blaise Cendrars, Pierre Reverdy, Alexandra Exter, Jean Cocteau, Erik Satie, Ossip Zadkine, Marc Chagall, and Marcel Duchamp. In 1911, he joined the Puteaux Group, a branch of the cubist movement.

On 7 September 1911, police arrested and jailed him on suspicion of stealing the Mona Lisa, but released him a week later. Apollinaire then implicated his friend Pablo Picasso, who was also brought in for questioning in the art theft, but he was also exonerated. He once called for the Louvre to be burnt down.

He fought in World War I and, in 1916, received a serious shrapnel wound to the temple. He wrote Les Mamelles de Tirésias while recovering from this wound. During this period he coined the word surrealism in the program notes for Jean Cocteau and Erik Satie's ballet Parade, first performed on 18 May 1917. He also published an artistic manifesto, L'Esprit nouveau et les poètes. Apollinaire's status as a literary critic is most famous and influential in his recognition of the Marquis de Sade, whose works were for a long time obscure, yet arising in popularity as an influence upon the Dada and Surrealist art movements going on in Montparnasse at the beginning of the twentieth century as, "The freest spirit that ever existed."

The war-weakened Apollinaire died of influenza during the Spanish Flu pandemic of 1918. He was interred in the Père Lachaise Cemetery, Paris.

Apollinaire's first collection of poetry was L'enchanteur pourrissant (1909), but Alcools (1913) established his reputation. The poems, influenced in part by the Symbolists, juxtapose the old and the new, combining traditional poetic forms with modern imagery. In 1913, Apollinaire published the essay Les Peintres cubistes on the cubist painters, a movement which he helped to define. He also coined the term orphism to describe a tendency towards absolute abstraction in the paintings of Robert Delaunay and others.

In 1907, Apollinaire wrote the well-known erotic novel, The Eleven Thousand Rods (Les Onze Mille Verges). Officially banned in France until 1970, various printings of it circulated widely for many years. Apollinaire never publicly acknowledged authorship of the novel. Another erotic novel attributed to him was The Exploits of a Young Don Juan (Les exploits d'un jeune Don Juan), in which the 15 year old hero fathers three children with various members of his entourage, including his aunt. The book was made into a movie in 1987.

Shortly after his death, Calligrammes, a collection of his concrete poetry (poetry in which typography and layout adds to the overall effect), and more orthodox, though still modernist poems informed by Apollinaire's experiences in the First World War and in which he often used the technique of automatic writing, was published.

In his youth Apollinaire lived for a short while in Belgium, mastering the Walloon dialect sufficiently to write poetry through that medium, some of which has survived.