A facile method has been developed for face-to-face assembly of two-dimensional surfactant-free Pd nanosheets into one-dimensional Pd superlattice nanowires. The length of the Pd nanowires can be well controlled by introducing cations of different concentration and charge density. Our studies reveal that cations with higher charge density have stronger charge-screening ability, and their introduction leads to more positive zeta-potential and decreased electrostatic repulsion between negatively charged Pd nanosheets. Moreover, their surfactant-free feature is of great importance in assembling the Pd nanosheets into superlattice nanowires. While the cations are important for the assembly of Pd nanosheets, the use of poly(vinylpyrrolidone) is necessary to enhance the stability of the assembled superlattice nanowires. The as-assembled segmented Pd nanowires display tunable surface plasmon resonance features and excellent hydrogen-sensing properties.