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Extracellular biosynthesis of silver nanoparticles (AgNPs) was explored using Thiosphaera pantotropha since this strain exhibits both nitrate- and nitrite-reductase enzyme activity (NaR and NiR, respectively). Optimal AgNP synthesis was achieved using 2 mM AgNO₃, culture supernatant of nutrient broth grown T. pantotropha, and incubation at 37 °C and 180 rpm. Under these conditions, the localized surface plasmon resonance peak of silver at 404 nm matched well with the average size of the spherical AgNPs based on FEG-TEM micrographs, i.e., 14.6 nm (range: 5–51 nm). The zeta potential of −33.6 mV indicated good stability of the biosynthesized nanoparticles. The XRD spectra demonstrated the simultaneous presence of face-centered cubic crystal structure of AgNPs and AgCl NPs. Ag⁺ ions were possibly reduced by the NaR and NiR enzymes released into the culture media. The FTIR spectra confirmed …