Carl Kellner (1 September 1851 – June 7, 1905) was a chemist, inventor, and industrialist. Born in Vienna, Austria, he made significant improvements to the sulfite process and was co-inventor of the Castner-Kellner process. He was a student of Freemasonry, Rosicrucianism, and Eastern mysticism. He was the putative founder of Ordo Templi Orientis and is sometimes referred to as "Spiritual father of O.T.O.".