The bioherbicidal activity of kapa-kapa (Medinilla magnifica Lindl.) was studied for the first time. Phenolics from its leaves were extracted with 7:7:6 methanol-acetone-water (MAW). The phenolics of the crude extract were quantified using the Folin-Ciocalteu method and values of 71.86 mg GA∙g-1 extract for total phenolics and 29.58 mg QUE∙g-1 extract for total flavonoids were obtained. The crude extract was subjected to acid hydrolysis and the bioherbicidal activity of the resulting hydrolysate was determined. Lettuce seed germination assay was done and the median lethal dose (LD50) of the extract was determined as 64.69 ppm. This relatively low LD50 value shows that it has great potentials for use against weeds. The extract inhibited the growth of Echinochloa crus-galli, Cyperus iria, and Ludwigia hyssopifolia, with E. crus-galli showing the greatest sensitivity among the three species. Total chlorophyll content of soybean was reduced by the extract. This suggests that the acid-hydrolyzed extract of M. magnifica decreased chlorophyll production, resulting in reduced biomass of the test weeds.