(b Pyritz [now Pyrzyce, Poland], 14 March 1845; d Berlin, 9 Nov 1912). German musicologist. He studied music with Heinrich Bellermann, and history, at the University of Berlin (1863–70), taking the doctorate in 1870 with a dissertation on mensural notation in the 12th and 13th centuries; he completed the Habilitation in 1872 at the newly founded Strasbourg University and taught there as an external lecturer. In 1875 he was appointed reader and organized a department which included an extensive research library. At that time he was also conductor of the Akademische Gesangsverein and composed numerous a cappella works intended for liturgical as well as concert performance. From 1897 to 1905 he was professor of musicology, the only person to hold such a post at a German university at the time.
In his writings Jacobsthal concentrated mainly on the music of the Middle Ages; his chief work deals with chromatic alteration in Western chant. His studies of Palestrina's works reveal them as a source for German Romanticism and identify their value in the context of 16th-century polyphonic style. These ideas were not recognized until the 1920s, when they established a new musicological perspective. Jacobsthal developed an approach to musicology which used research methods from history and philology, thereby paving the way for the research of medieval music undertaken by his pupils Friedrich Ludwig and Peter Wagner.