The plasmon-mediated synthesis of silver nanoparticles is a versatile synthetic method which leverages the localized surface plasmon resonance (LSPR) of nanoscale silver to generate particles with non-spherical shapes and control over dimensions. Herein, a method is reported for controlling the twinning structure of silver nanoparticles, and consequently their shape, via the plasmon-mediated synthesis, solely by varying the excitation wavelength between 400, 450, and 500 nm, which modulates the rate of Ag+ reduction. Shorter, higher energy excitation wavelengths lead to faster rates of reaction, which in turn yield structures containing a greater number of twin boundaries. With this method, silver cubes can be synthesized using 450 nm excitation, which represents the first time this shape has been realized by a plasmon-mediated synthetic approach. In addition, these cubes contain an unusual twinning structure composed of two intersecting twin boundaries or multiple parallel twin boundaries. With respect to their twinning structure, these cubes fall between planar-twinned and multiply twinned nanoparticles, which are synthesized using 500 and 400 nm excitation, respectively.