Diarrhea is among the leading cause of death especially among children aged under 5 years. Zinc has been included in the management and treatment of diarrhea. Nanoparticle is gaining importance in medicine. The aim of this study is to evaluate the antidiarrheal activity of biosynthesized zinc oxide nanoparticle in castor oil-induced diarrheal rats. Zinc oxide nanoparticle (ZnONP) was biosynthesized using Cassia occidentalis leaf. The biosynthesized ZnONP was characterized using ultraviolet (UV)-spectroscopy and Fourier transform infrared spectroscopy (FTIR). Twenty five albino rats were randomly grouped into five groups. All rats, except the control, were induced with diarrhea. Group1 received normal saline, group 2 received 3 mg/kg b.wt. loperamide while groups 3, 4 and 5 received 10, 20, and 50 mg/kg b.wt. biosynthesized ZnONP. UV spectroscopy showed an absorption peak at 320 nm. The SEM micrographs reveal the nanoparticles as flake like which are well agglomerated.. The SEM micrographs reveal the nanoparticles as flake like which are well agglomerated. The biosynthesized zinc oxide nanoparticle significantly delayed the onset of diarrhea, decreased the number of wet feces, increased the percentage gastrointestinal tract (GIT) inhibition and decreased the volume of intestinal fluid. Biosynthesized zincoxide nanoparticle from Cassia occidentalis leaf can be used in the treatment of diarrhea.