Poly(amino acid) (PAA)-based nanocomposites were synthesized by in situ melting polycondensation with addition of different contents of ZnO nanoparticles (ZnO NPs); the morphology and thermal, mechanical, cytotoxicity, antibacterial and dielectric properties of the resulting nanocomposites were investigated and characterized. The nanoparticles were dispersed within the matrix without the need for coupling agent and surfactants. The crystallization temperature had a gradual rise with increasing ZnO NP loading, confirming that the nanoparticles act as nucleating agents for PAA crystallization. The nanoparticles increased the thermal stability of the matrix. The mechanical properties were gradually enhanced by increasing the ZnO NP content except for the elongation at break, which showed its highest value at 1 wt% ZnO NP contents. They also exhibited active inhibition against both Gram-positive and Gram-negative bacteria, which was gradually enhanced with increase in ZnO NP content. Meanwhile, the nanocomposites showed no obvious cytotoxicity. Thus, these nanocomposites are suitable as performance materials in the field of medical apparatus.