Hans Leo Haßler (baptized October 26, 1564 ? June 8, 1612) was a German composer and organist of the late Renaissance and early Baroque eras. He was born in Nuremberg and died in Frankfurt am Main.
Hassler most likely studied music as a boy with his father, an organist. In 1584, like many German composers of the time, he went to Italy to continue his studies; he arrived in Venice during the peak of activity of the Venetian school, the composers who wrote in the resplendent polychoral style, which was soon to become popular outside of its native city. Hassler was already familiar with some of this music, as numerous prints had circulated in Germany due to the interest of Leonhard Lechner, who was associated with Orlandus Lassus in Munich.
While in Venice, Hassler became friends with Giovanni Gabrieli; together they studied with Andrea Gabrieli, Giovanni's uncle.
Hassler returned to Germany in 1586, moving to Augsburg where he served as an organist to Octavian II Fugger, a nobleman there. The Augsburg years were extremely creative for him; in addition he became well-known as a composer and organist at this time, though his influence was limited because he was a Protestant in an area which was still heavily Catholic. Later he moved to Nuremberg, and then to Ulm; around this time he acquired tuberculosis, which proved to be fatal, although he did have a brief stint as chapel organist in Dresden, where he composed his last works. After he died, Michael Praetorius and Heinrich Schütz were appointed in his place.