Thelonious Sphere Monk (/θəˈloʊniəs/ October 10, 1917 – February 17, 1982) was an American jazz pianist and composer. He had a unique improvisational style and made numerous contributions to the standard jazz repertoire, including "'Round Midnight", "Blue Monk", "Straight, No Chaser", "Ruby, My Dear", "In Walked Bud", and "Well, You Needn't". Monk is the second-most-recorded jazz composer after Duke Ellington.

Monk's compositions and improvisations feature dissonances and angular melodic twists, often using flat ninths, flat fifths, unexpected chromatic notes together, low bass notes and stride, and fast whole tone runs, combining a highly percussive attack with abrupt, dramatic use of switched key releases, silences, and hesitations.

Monk's distinct look included suits, hats, and sunglasses. He also had an idiosyncratic habit during performances: while other musicians continued playing, Monk would stop, stand up, and dance for a few moments before returning to the piano.

Monk is one of five jazz musicians to have been featured on the cover of Time (the others being Louis Armstrong, Dave Brubeck, Duke Ellington, and Wynton Marsalis).