Leonard Joseph Tristano (March 19, 1919 – November 18, 1978) was an American jazz pianist, composer, arranger, and teacher of jazz improvisation.

Tristano studied for bachelor's and master's degrees in music in Chicago before moving to New York City in 1946. He played with leading bebop musicians and formed his own small bands, which soon displayed some of his early interests – contrapuntal interaction of instruments, harmonic flexibility, and rhythmic complexity. His quintet in 1949 recorded the first free group improvisations. Tristano's innovations continued in 1951, with the first overdubbed, improvised jazz recordings, and two years later, when he recorded an atonal improvised solo piano piece that was based on the development of motifs rather than on harmonies. He developed further via polyrhythms and chromaticism into the 1960s, but was infrequently recorded.

Tristano started teaching music, especially improvisation, in the early 1940s, and by the mid-1950s was concentrating on teaching in preference to performing. He taught in a structured and disciplined manner, which was unusual in jazz education when he began. His educational role over three decades meant that he exerted an influence on jazz through his students, including saxophonists Lee Konitz and Warne Marsh.

Musicians and critics vary in their appraisal of Tristano as a musician. Some describe his playing as cold and suggest that his innovations had little impact; others state that he was a bridge between bebop and later, freer forms of jazz, and assert that he is less appreciated than he should be because commentators found him hard to categorize and because he chose not to commercialize.