Per Teodor Cleve (10 February 1840 – 18 June 1905) was a Swedish chemist, biologist, mineralogist and oceanographer.

[1][2] Born in Stockholm in 1840, Cleve earned his BSc and PhD from Uppsala University in 1863 and 1868, respectively.

In 1874 he theorised that didymium was in fact two elements; this theory was confirmed in 1885 when Carl Auer von Welsbach discovered neodymium and praseodymium.

[4][5] Cleve's ancestors on his father's side came from western Germany and settled in Sweden in the late 18th century.

[1][6] He also taught at the Royal Institute of Technology between 1870 and 1874, and eventually became professor of general and agricultural chemistry at the University of Uppsala.

This theory was proven right with the discovery of praseodymium and neodymium in 1885 by Carl Auer von Welsbach.

[4] In 1890, Cleve began to mainly focus on the field of biology, mainly studying freshwater algae, diatoms, and plankton.

[15] With Johann Diedrich Möller he issued and distributed an exsiccata-like series of microscope slides under the title Diatoms edited by P. T. Cleve and J. D.

[16] Cleve, in collaboration with Otto Höglund prepared numerous previously-undiscovered salts of yttrium and erbium.

[7] Additionally, Cleve created a method of dating glacial and post-glacial deposits in the fossil record.

Notable students of Cleve include Ellen Fries (the first Swedish woman to earn a PhD) and Svante Arrhenius (a winner of the Nobel Prize).

The third and last daughter, Célie Brunius (born Gerda Cecilia Afrodite Cleve) (1882-1980), was a journalist.

His daughter Agnes was married to illustrator, set designer and artist John Jon-And.

Per Teodore Cleve was a supporter of women's equality and Ellen Fries, the first Swedish woman to receive a PhD, was one of his students.