This research work investigates on the treatment of water being contaminated by carcinogenic dye and bacteria using tin oxide (SnO₂) nanoparticles synthesized by sol–gel process by analysing their structural, optical, morphological and thermal properties through X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy (UV–Vis), photoluminescence spectroscopy (PL), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and thermogravimetric (TGA–DTA) analysis. The specific surface area and pore size distribution have been determined using Brunauer–Emmett–Teller (BET) surface area analysis. The utilisation of SnO₂ nanoparticles as photocatalysts for the degradation of the Congo Red (C₃₂H₂₂N₆Na₂O₆S₂) dye(CR) and also as anti-bacterial material for bacteria such as Bacillus sp., Pseudomonas sp. and E. coli were examined. The effects of SnO₂ nanoparticles as photocatalysts for the Congo Red dye have been explored under different solar irradiations, i.e., on bright sunny days (direct radiation), cloudy days (diffused radiation) and also under ultra-violet [UV (312 nm)] radiation and the degradation efficiency was analysed and illustrated. The highest degradation efficiency of about 94% obtained for the as-prepared sample which was kept under bright sunlight was due to the increase in surface area, mesoporosity and effective charge separation of photogenerated electron–hole pairs. The antimicrobial activity of the as-synthesised SnO₂ nanoparticle has also been investigated under six different concentrations. The results showed that increase in the concentration increases the antibacterial activity which was due to the increase in the H₂O₂ formation from the surface of SnO₂ nanoparticle. Thus, this work explicated that the as-prepared SnO₂ nanopowder can be effectively used in the treatment of water systems affected by carcinogenic dye or bacteria.