Plinia cauliflora (Mart.) Kausel, widespread in South America, has edible fruits, and its bark is commonly used against diarrhea and other disorders, on account of its astringency. Because diarrhea is still one of the most important causes of illness and death among children in developing countries, where the population turns to traditional medicine for its treatment, the present study determined the composition of fruit and leaf extracts of P. cauliflora, analyzed the activity against diarrhea by antimicrobial and gastrointestinal motility, and evaluated the cytotoxicity of the extracts. Chemical composition was determined by high-performance liquid chromatograpy-ultraviolet/photodiode array detection. Antimicrobial activity was analyzed by agar diffusion and the microdilution method against etiological agents of diarrhea. The effect on gastrointestinal motility was analyzed using an experimental model in mice. Cytotoxicity was evaluated in vitro with the fibroblast cell line SIRC CCL 60, and leaf extract showed a 50% inhibitory concentration of 0.48 μg/mL. Gallic acid, ellagic acid, and flavonoid derivatives were detected in the extracts. It was observed that fruit and leaf extracts showed some activity against Enterococcus faecalis, Escherichia coli, Salmonella sp., and Shigella sp. However, neither extract had any effect on gastrointestinal motility.