Ernst Wilhelm Oskar Windisch (4 September 1844, Dresden – 30 October 1918, Leipzig) was a German scholar and Celticist. He is known as an Indo-Europeanist. He was a son-in-law to economist Wilhelm Roscher. He was also a friend of the young Friedrich Nietzsche.
In 1867 he obtained his PhD in classical philology at the University of Leipzig, afterwards remaining in Leipzig as a teacher at the "Thomasschule" (1867–1870). In the meantime, he received his habilitation in Sanskrit and comparative linguistics at the university (1869).
In 1870–71 he worked as a staff member of the India Office Library in London. Later on, he became a professor at the University of Leipzig, where in 1895/96 he served as rector. From 1883 to 1918, he was a member of the Sächsische Akademie der Wissenschaften zu Leipzig (Royal Saxon Society of Sciences in Leipzig).
- Irische Texte mit Wörterbuch, Leipzig 1880 – Irish texts with dictionary.
- "Compendium of Irish Grammar" (1883 English translation).
- Zwölf Hymnen des Rigveda, mit Sayana's Commentar (1883)
- Irische Texte, 4 vols. (1880-1909) with Whitley Stokes
- Māra und Buddha, Leipzig 1895.
- Buddhas Geburt und die Lehre von der Seelenwanderung, Leipzig 1908 – Buddha's birth and the doctrine of the transmigration of souls.
- Iti-Vuttaka, editor
- Das keltische Britannien bis zu Kaiser Arthur, Leipzig 1912 – Celtic Britain up to the time of King Arthur.
- Festschrift (1914).
- Geschichte der Sanskrit-Philologie und indischen Altertumskunde, 2 vols, Leipzig 1917-1920 – History of Sanskrit philology and Indian archaeology.
- Kleine Schriften (2001) edited by Karin Steiner and Jörg Gengnagel.