**Horst Sachs** (27 March 1927 – 25 April 2016) was a German mathematician, an expert in graph theory, a recipient of the Euler Medal (2000).^{[1]}

He earned the degree of Doctor of Science (Dr. rer. nat.) from the Martin-Luther-Universität Halle-Wittenberg in 1958.^{[2]} Following his retirement in 1992, he was professor emeritus at the Institute of Mathematics of the Technische Universität Ilmenau.^{[3]}

His encyclopedic book in spectral graph theory, *Spectra of Graphs. Theory and Applications* (with Dragos Cvetković and Michael Doob) has several editions and was translated in several languages.^{[4]}^{[5]}^{[6]}^{[7]}^{[8]}^{[9]}

Two theorems in graph theory bear his name. One of them relates the coefficients of the characteristic polynomial of a graph to certain structural features of the graph. Another one is a simple relation between the characteristic polynomials of a graph and its line graph. Sachs subgraphs are also named after Sachs.