Walter Hamor Piston Jr. (January 20, 1894 – November 12, 1976) was an American composer.

He was born in Rockland, Maine. His father's father, the sailor Antonio Pistone changed his name to Anthony Piston when he came to America from Genoa, Italy. In 1905, Walter Piston Sr. and his family moved to Boston. Walter Jr. trained as an engineer at the Mechanical Arts High School in Boston, but he was artistically inclined and upon graduating from there in 1912, proceeded to the Massachusetts Normal Arts School, majoring in painting, also studying architectural drawing and American history. There he met Kathryn Nason, and married her at an Unitarian church.

With his brother Edward, Walter Piston Jr. took piano lessons from Harris Shaw (who was Virgil Thomson's organ teacher). During the 1910s Walter Piston made a living playing piano and violin in dance bands, and later on in the decade played violin in orchestras led by Georges Longy. With help from Shaw, Walter Piston was admitted to Harvard in 1920, where he studied counterpoint with Archibald Davison, canon and fugue with Clifford Heilman, advanced harmony with Edward Ballantine, composition and music history with Edward Burlingame Hill. Piston often worked as an assistant to the various music professors there, and conducted the student orchestra.

At about that time Piston joined the Navy Band and learned to play more instruments. He wanted to join the U.S. Navy as an officer, but was deemed more useful as a musician.

Upon graduating summa cum laude from Harvard, Piston was awarded a John Knowles Paine Traveling Fellowship, consisting of $1500 yearly for two to three years of travel abroad. He chose to go to Paris, living there from 1924 to 1926, but he also visited Italy. At the Ecole Nationale de Musique in Paris, Piston studied composition and counterpoint with Nadia Boulanger, composition with Paul Dukas and violin with George Enescu. His Three Pieces for Flute, Clarinet and Bassoon of 1925 made up his first published score.

He moved to Belmont, Massachusetts after returning from Europe, and taught at Harvard from 1926 until retiring in 1960. His students include Samuel Adler, Leroy Anderson, Arthur Berger, Leonard Bernstein, Elliott Carter, Irving Fine, John Harbison, Frederic Rzewski and Harold Shapero.

In 1936, the Columbia Broadcasting System commissioned six American composers (Aaron Copland, Louis Gruenberg, Howard Hanson, Roy Harris, William Grant Still and Piston) to write works for CBS radio stations to broadcast. Piston considered radio better suited to smaller orchestras, thus he wrote a Concertino for Piano and Chamber Orchestra. The following year Piston wrote his Symphony No. 1, which was premiered by the Boston Symphony Orchestra on April 8, 1938.

At the invitation of Arthur Fiedler, Piston wrote his most famous ballet, The Incredible Flautist, for Hans Wiener and the Boston Pops Orchestra.

Piston studied the twelve-tone techniques of Arnold Schoenberg, and wrote a work for organ using them, the Chromatic Study on the Name of Bach.

During World War II, Piston was an air raid warden in Belmont, and he wrote patriotic fanfares and other such works.

In 1943, the Alice M. Ditson fund of Columbia University commissioned Piston's Symphony No. 2, which was premiered by the National Symphony Orchestra on March 5, 1944 and was awarded a prize by the New York Music Critics' Circle. His next symphony, Symphony No. 3 earned a Pulitzer Prize, as did his Symphony No. 7. His Viola Concerto and String Quartet No. 5 also later received Critics' Circle awards.

Piston wrote three books, Counterpoint, Orchestration and Harmony. The last of these went through four editions in the author's lifetime, was translated to several languages, and even today (with changes made by a later author) is considered useful to teachers and students of harmony. Piston's handwriting was so neat that almost all his orchestral scores were published as facsimiles of his original scores, and he also wrote the musical examples in the textbooks he authored.

In his final years, Piston was debilitated by diabetes, and his vision and hearing suffered. His wife died in 1976, and he died later that same year, of a heart attack, in Belmont, Massachusetts. He was cremated, and his ashes were dispersed at Mt. Auburn Cemetery.