Werner Stumm (1924 – 14 April 1999) was a Swiss chemist. After earning his doctorate in inorganic chemistry at the University of Zürich in 1952 he moved to the U.S. where he was active as a professor at Harvard University until 1969. From 1970 until 1992 he was head of the Swiss Federal Water Resources Centre EAWAG.

Werner Stumm was an active researcher in several aspects of geochemistry. During the early part of his career he was influenced by the ideas of Lars Gunnar Sillén and Robert Garrels regarding aqueous chemical equilibria. He developed models where the ideas by Sillén regarding equilibria were combined with improved descriptions of kinetically controlled reactions (i.e. slow reactions that do not reach equilibrium, e.g. weathering). He in particular made contributions to the knowledge of the reactions at the interface of minerals and water. In his more than 200 scientific articles he showed, among other things, that the rate by which a mineral weathers depends on the surface charge, which in turn is controlled by factors such as pH and the chemical composition of aqueous solutions.

Stumm is also known for writing the influential book Aquatic chemistry, together with James J. Morgan, and several other books on the interplay between mineral surfaces and water.

The European Association of Geochemistry quinquennially awards a Science Innovation Award medal named in his honour for work in low-temperature and surface geochemistry.