Photocatalysts are a boon to the emerging pollution and environmental hazards posed due to the use of dyes in industries. Existing conventional methods are sometimes expensive and might release toxic by-products. A much greener approach to the challenge is the usage of nanoparticles which is a much safer and an environment friendly method to degrade the harmful and toxic chemicals. Hence, in the present study we have come up with a greener way of synthesizing spherical shaped tin oxide nanoparticles (SnO2 NPs) as a photocatalyst, utilizing Arachis hypogaea (A. hypogaea). The methanolic extract of A. hypogaea shell was used for preparation of SnO2 NPs having an average particle size of 60 nm, which was then used as the catalyst for Congo red dye degradation. The effect of dye concentration, catalyst concentration and irradiation time on the percentage of degradation and the optimization of the process was studied by Response Surface Methodology (RSM) based on Box–Behnken design. The obtained RSM model with R2 = 98.60% showed a satisfactory correlation between the predicted and experimental values of dye removal. 89% degradation was achieved under the optimum condition (1 mg photocatalyst concentration, 1 mM dye concentration) after 50 min time. In addition, the antifungal activity of the prepared nanoparticles was also carried out and revealed better results against Aspergillus niger (A. niger) than Aspergillus flavus (A. flavus), at higher concentration of SnO2 NPs. Thus we were able to synthesize a photocatalyst using an agricultural waste which can degrade an azo dye and also exhibits antifungal activity. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.