The noble metal-based nanoparticles (NPs) have been considered as potential antimicrobial agents because of their good antibacterial and antifungal activities as well as biocompatible nature. In this study, we have introduced a simple and fast route to synthesize silver, gold, and silver-gold bimetallic NP-decorated dextran. The as-synthesized noble metal-based NPs with spherical geometry showed high dispersity in dextran. The antibacterial and antifungal of obtained nanomaterials were tested with Xanthomonas oryzae pv. oryzae (Xoo) bacteria and Magnaporthe grisea (M. grisea) fungi. The silver NPs and bimetallic NPs with high silver content in dextran exhibited excellent activity to inhibited the growth of the bacteria and fungi, whereas the gold/dextran has weak antimicrobial effects. The antibacterial and antifungal properties of silver-gold bimetallic NPs in dextran biopolymer can be tuned according to the content of silver in the bimetallic NPs. The obtained nanomaterials could open an entry to a new class of antibiotics.