Hermann Osthoff (18 April 1847, Billmerich – 7 May 1909, Heidelberg) was a German linguist. He was involved in Indo-European studies and the Neogrammarian school. He is known for formulating Osthoff's law, and published widely on Indo-European word-formation and morphology.
Osthoff studied classical philology, Germanic philology, Sanskrit and comparative linguistics in Berlin, Tübingen and Bonn. In 1869 he obtained his doctorate in Bonn as a student of Hermann Usener. During his time in that city he became a member of the Burschenschaft Alemannia of Bonn. From 1871 onward, he taught classes at the gymnasium in Kassel. In 1875, he successfully completed his postdoctoral habilitation at the University of Leipzig, and in 1877, was named an associate professor of comparative linguistics and Sanskrit at the University of Heidelberg. Shortly afterwards, he was granted full professorship at Heidelberg, where he later served as dean (1894/95) and vice-rector (1899–1900).
The main focus of his research was in Indo-European languages. Along with Karl Brugmann and August Leskien, he was a significant figure in the founding of the Neogrammarians. He was a vocal critic of a young Ferdinand de Saussure's pioneering work Mémoire sur le système primitif des voyelles dans les langues indo-européennes, in which the 20-year-old Saussure postulated the laryngeal theory in 1879. However, Saussure's theory was confirmed with the discovery of Hittite in the early 20th century.