Benjamin Burwell Johnston Jr. (March 15, 1926 – July 21, 2019) was an American contemporary music composer, known for his use of just intonation. He was called "one of the foremost composers of microtonal music" by Philip Bush and "one of the best non-famous composers this country has to offer" by John Rockwell.

Johnston was born in Macon, Georgia, and taught composition and theory at the University of Illinois at Urbana–Champaign from 1951 to 1986, before retiring to North Carolina. During his time teaching, he was in contact with avant-garde figures such as John Cage, La Monte Young, and Iannis Xenakis. Johnston's students included Stuart Saunders Smith, Neely Bruce, Thomas Albert, Michael Pisaro, Manfred Stahnke, and Kyle Gann. He also considered his practice of just intonation to have influenced other composers, including Larry Polansky. In 1946 he married dance band singer Dorothy Haines, but they soon divorced. In 1950 he married artist Betty Hall, who died in 2007.

Johnston began as a traditional composer of art music before working with Harry Partch. He helped the senior musician to build instruments and use them in the performance and recording of new compositions. Partch then arranged for Johnston to study with Darius Milhaud at Mills College. In 1952, Johnston met Cage, who invited him to come to New York to study with him in the summer. Though Johnston decided he did not have sufficient time to prepare for such studies, he did go to New York for several weeks and assisted, along with Earle Brown, in the production of Cage's eight-track tape composition, Williams Mix.

Later, in 1957 and 1959, he studied with Cage, who encouraged him to follow his desires and use traditional instruments rather than electronics or newly built instruments. Unskilled in carpentry and finding electronics unreliable, Johnston struggled with how to integrate microtonality and conventional instruments for ten years. He also struggled with how to integrate microtones into his compositional language through a slow process of many stages. However, since 1960 Johnston had almost exclusively used a system of microtonal notation based on the rational intervals of just intonation, what Gann describes as a "lifelong allegiance" to microtonality. Johnston also studied with Burrill Phillips and Robert Palmer.

Johnston composed music for multiple productions by the E.T.C. Company of La MaMa, Wilford Leach and John Braswell's company-in-residence at La MaMa Experimental Theatre Club in the East Village of Manhattan. His most significant work was Carmilla, which the company performed as part of their repertory throughout the 1970s. He also composed music for the company's production of Gertrude, a musical about the life of Gertrude Stein.

His other works included the orchestral work Quintet for Groups (commissioned by the St. Louis Symphony Orchestra), Sonnets of Desolation (commissioned by the Swingle Singers), the Sonata for Microtonal Piano (1964), and the Suite for Microtonal Piano (1977). Johnston completed ten string quartets. The Kepler Quartet recorded all ten of his string quartets for New World Records, finishing in April 2016 just after the composer's 90th birthday.

Johnston said:

Tempered tuning is not the acoustically simplest kind. In just tuning, any interval is tuned so as to eliminate 'beating' (the result of vibrations interfering with each other). Just intonation is the easiest to achieve by ear. In this kind of tuning, all intervals have vibration rates related by small whole-number ratios. The larger the integers of the ratio, the greater the dissonance.