Per Teodor Cleve (10 February 1840 – 18 June 1905) was a Swedish chemist, biologist, mineralogist and oceanographer. He became a student at Uppsala University in 1858 and earned his undergraduate and doctoral degrees in 1963 and 1968 respectively. After obtaining his PhD he became assistant professor of chemistry and later became professor of general and agricultural chemistry at Uppsala University. He was chair of chemistry at the university starting 1874, and became president of the Nobel Committee for Chemistry in 1900 until his death in 1905. Notable students of Cleve include Ellen Fries, the first Swedish woman to obtain a PhD, and Svante Arrhenius, a winner of the Nobel Prize in Chemistry in 1903.
Cleve is best known for his discovery of the chemical elements holmium and thulium in 1879. He discovered the brown and green substances, holmium oxide and thulium oxide respectively, while removing impurities from a sample of erbium oxide. His other contributions to chemistry include determining the atomic weight of the element helium, when together with Abraham Langlet, he discovered helium in the mineral cleveite (named after Cleve) in 1895. Moreover, he discovered six forms of dichloronaphthalene and discovered aminonaphthalenesulfonic acids, also known as Cleve’s acids.