Purpose This study aims to report the phytochemical screening and anti-diarrhoea properties of methanol extract of Diodia sarmentosa Swartz (DSS) leave (at 200, 400 and 600 mg/kg) in albino rats. Design/methodology/approach Three types of diarrhoea models (castor oil-induced diarrhoea, castor oil-induced gastrointestinal motility and castor oil-induced enteropooling), as well as an assay for serum and faecal electrolyte concentrations, were used. Acute toxicity study and phytochemical screening of the extract were carried out using standard techniques. Findings The percentage inhibition of diarrhoea by DSS 200, 400 and 600 mg/kg was obtained as 25.7%, 55% and 84.6%, respectively, compared with loperamide that had 87.6% inhibition. DSS dose dependently modulated the number and frequency of defecation, the weight of the faeces of the diarrhoea rats, the distance that was travelled by charcoal meal, mean differences in the full lengths of the intestines and the distance travelled by charcoal meal, weight and volume of intestinal contents, serum and faecal concentrations of potassium, sodium, chloride, magnesium, bicarbonate, iron and zinc relative to the negative control and in a manner akin to loperamide (for the extract at 600 mg/kg). DSS or loperamide attenuated the loss of calcium in the faeces of the rats. DSS possesses anti-diarrhoea properties, which may be attributed to the phytochemicals in it. Finally, the study showed the safety in the usage of DSS. Originality/value DSS exerted its anti-diarrhoea action in castor oil-induced diarrhoea rats, by modulating their intestinal secretory and motile activity in a manner akin to loperamide (for DSS at 600 mg/kg).