Thomas Hermann Geisser (born February 28, 1966) is a German mathematician working at Rikkyo University in Tokyo, Japan. He works in the field of arithmetic geometry, motivic cohomology and algebraic K-theory.
From 1985 Geisser studied at Bonn University under the supervision of Günther Harder and obtained a master's degree in 1990. He continued to obtain a PhD under the supervision of Christopher Deninger at the University of Münster; the title of his thesis is "A p-adic analog of Beilinson's conjecture for Hecke characters of imaginary quadratic fields".
Geisser spent three years at Harvard University as a visiting scholar and visiting fellow, respectively. After further stays in Essen, University of Illinois, Urbana-Champaign and Tokyo University, he became assistant professor at the University of Southern California, and was promoted to associate professor in 2002 and professor in 2006.
After visiting Tokyo University again, he became a professor at Nagoya University in 2010, and moved to Rikkyo University in 2015.
He received a Sloan Research Fellowship (2000) and a Humboldt Prize (2021).
He is editor for Documenta Mathematica and managing editor for Commentarii Mathematici Universitatis Sancti Pauli.