Considering that guanidine-based derivatives are good DNA minor groove binders, we have theoretically studied, using the Polarizable Continuum model mimicking water solvation, the complexes formed by the biologically relevant guanidinium cation and the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine, and uracil). The interactions established within these complexes both by hydrogen bonds and by cation-π interactions have been analyzed by means of the Atoms in Molecules and Natural Bond Orbital approaches. Moreover, maps of electron density difference have been produced to understand the cation-π complexes. Finally, the NICS and three-dimensional NICS maps of the cation-π complexes have been studied to understand the effect of the guanidinium cation on the aromaticity of the nucleobases.