Nanotechnology has become the most promising area of research with its application in various fields of science such as solar cell. Metals and metal oxide nanoparticles (NPs) are mainly synthesized by chemical methods that have unintended effects such as environmental pollution, large energy consumption and potential health problems. In response to these challenges, green synthesis, which uses plant extracts instead of industrial chemical agents to reduce metal ions, has been developed. In this study, nanocrystalline tin oxide (SnO2) NPs were synthesized by green method using Psidium guajava extract as a capping agent. The synthesized NPs were characterized using X-ray diffractometer (XRD), Ultra-violet visible spectrometer (UV-Vis) and photoluminescence spectroscopy. The XRD analysis revealed that synthesized SnO2 nanoparticles have tetragonal rutile structure. Crystallite sizes were found to be 14.89 and 18.00 nm estimated by using Debye-Scherrer’s equation for capped and uncapped SnO2 NPs the capping agent respectively. Photoluminescence study of prepared SnO2 NPs excited at 260 nm showed emission peak at 472.67 nm for capped and 353.32, 378.09 394.21 and 472.67 nm uncapped SnO2.UV-Vis spectroscopy was used to determine the absorbance which was found to be 308 (capped) and 290 nm (uncapped) SnO2NPs. The band gap energy was estimated using Tauc's equation and found to be 3.98 and 3.21 eV for capped and uncapped SnO2 NPs respectively. Fourier transform infrared spectroscopy showed the stretching vibration of Sn-O at 749.23 and 746.44 cm-1 wavenumbers for capped and uncapped SnO2 respectively. This study successfully demonstrates the green synthesis of tin oxide (SnO2) nanoparticles using Psidium guajava extract as a capping agent. The characterization results confirm the formation of tetragonal rutile SnO2 nanoparticles with varying crystallite sizes and optical properties between capped and uncapped samples. Further studies should focus on optimizing reaction conditions, such as extract concentration, temperature, and reaction time, to improve nanoparticle quality and yield.