Klaus Müllen (born 2 January 1947) is a German chemist working in the fields of polymer chemistry, supramolecular chemistry and nanotechnology. He is known for the synthesis and exploration of the properties of graphene-like nanostructures and their potential applications in organic electronics.

His research interests are in the field of preparative macro- and supramolecular chemistry. Among other things, his group has succeeded in synthesizing and characterizing hitherto unattainable large polycyclic aromatics such as superphenalene, which has a molecular mass of 1182 g·mol−1 and consists of 34 condensed benzene rings.

He has developed small disc-like organic building blocks using alkyl-substituted hexabenzocoronene, and in particular HBC-C12 – which self-assembles into crystalline liquid-phase structures (columnar liquid crystals) as potential organic field-effect transistors. The considered two-dimensional benzene ring structures are examples of subunits of graphene lattices (graphene nanostructures). The graphene-like structures synthesized and investigated by Müllen include two-dimensional bands of less than 50 nanometers width with jagged edges. Of interest here are the electronic conduction properties and spintronics properties with a view to future replacement of silicon-semiconductor technology. In synthesis, he introduced a new method in graphene polymer chemistry: soft-landing mass spectrometry. Applications include synthetic light-emitting organic materials (such as OLEDs) and incorporation of molecular defects (defect engineering) organic analogues of semiconductor technology.