Theo Geisel (born 24 August 1948 in Limburg an der Lahn, Hesse) is a German physicist.
Geisel is director at the Max Planck Institute for Dynamics and Self-Organization and professor of theoretical physics at the University of Göttingen.
His research is primarily concerned with the behavior of complex systems ranging from theoretical investigations in quantum chaos to nonlinear phenomena occurring in the brain.