Tetragonal SnO2 nanoparticles (1540 nm) were synthesized according to a green biological synthesis technique using Gram-negative bacteria Erwinia herbicola followed by an annealing treatment over 425 K. The SnO2 nanoparticles were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), and scanning electron microscopy with energy dispersive X-ray (SEM-EDX). The zeta potential of biosynthesized SnO2 nanoparticles was 7.53 mV. A biosynthesis mechanism for SnO2 nanoparticles was also proposed. In the biosynthesis, the bacterial protein and biomolecules served as the template for reduction and stabilization of SnO2 nanoparticles. These biomolecules also helped in controlling SnO2 nanoparticle size and aggregation. The SnO2 nanoparticles exhibited excellent photocatalytic activity for photodegradation of organic dyes such as methylene blue, methyl orange, and erichrome black T. Approximately 93.3, 97.8, and 94.0% degradations of methylene blue, erichrome black T, and methyl orange were observed with biosynthesized SnO2 nanoparticles in the photocatalytic degradation process, respectively.