We have demonstrated glutathione- and cysteine-induced transverse overgrowth on gold nanorods. In aqueous solutions, glutathione and cysteine are preferentially bound to the ends of gold nanorods that are stabilized by a cationic surfactant bilayer. This preferential end binding blocks the growth of the nanorods in the longitudinal direction completely and allows for the growth only in the transverse direction. As a result, the diameters of the nanorods become larger and larger while their lengths remain unchanged, as more and more gold precursor is supplied. In addition, the shape of the nanorods undergoes a gradual change from rods, peanuts, and truncated octahedra to faceted spheres during overgrowth. We believe that this transverse overgrowth provides an alternative means for tailoring the longitudinal plasmon wavelengths and extinction cross sections of gold nanoparticles and will therefore facilitate their use in optics, optoelectronics, and biotechnology.