Johann Michael Haydn (September 14, 1737 – August 10, 1806) was an Austrian composer, the younger brother of Joseph Haydn.

Michael Haydn was born in Rohrau, and like his brother, he was a chorister at St Stephen's in Vienna. Shortly after leaving the choir-school, he was appointed Kapellmeister at Großwardein and later, in 1762, at Salzburg. The latter office he held for forty-three years, during which time he wrote over 360 compositions for the church and much instrumental music. He was an intimate friend of Mozart, who had a high opinion of his work, and the teacher of Carl Maria von Weber.

Haydn's sacred choral works are generally regarded as being his most important, including the Missa Hispanica (which he exchanged for his diploma at Stockholm), a Mass in D minor, a Lauda Sion, and a set of graduals, forty-two of which are reprinted in Anton Diabelli's Ecciesiaslicon. He was also a prolific composer of secular music, including forty symphonies, a number of concerti and chamber music including a string quintet in C major which was once thought to have been by his brother Joseph.

Michael Haydn was the victim of another case of posthumous mistaken identity: for many years, the piece which is now known as Michael Haydn's Symphony No. 26 was thought to be Mozart's Symphony No. 37. The confusion arose because an autograph was discovered which had the opening movement of the symphony in Mozart's hand, and the rest in somebody else's. It is now thought that Mozart had composed a new slow opening movement for reasons unknown, but the rest of the work is known to be by Michael Haydn. The piece, which had been quite widely performed as a Mozart symphony, has been performed considerably less often since this discovery in 1907.