Friedhelm Waldhausen (born 1938 in Millich, Hückelhoven, Rhine Province, died 2024) was a German mathematician known for his work in algebraic topology.
He obtained his Ph.D. in 1966 from the University of Bonn; his advisor was Friedrich Hirzebruch and his thesis was entitled "Eine Klasse von 3-dimensionalen Mannigfaltigkeiten" (A class of 3-dimensional manifolds).
This led to new foundations for algebraic K-theory (using what are now called Waldhausen categories) and also gave new impetus to the study of highly structured ring spectra.
Today, Waldhausen is seen, together with Daniel Quillen, as one of the pioneers of algebraic K-theory.
Among others, he was awarded the von Staudt Prize in 2004 along with Günter Harder, and an honorary doctorate from the Universität Osnabrück.