Ageratina riparia (Regel) R.M. King and H. Rob. of the family Asteraceae is an invasive weed distributed widely in the montane regions of the Central Province of Sri Lanka. The present study was carried out to investigate the antifungal components of the weed, A. riparia and their ability to control banana anthracnose disease caused by the fungus, Colletotrichum musae. The methanol extract of the aerial part of A. riparia was subjected to activity guided fractionation and methylripariochromene A was isolated as the bioactive compound. The effectiveness of the methanol extract of A. riparia against banana anthracnose disease was assessed and a promising reduction of the anthracnose lesions was observed specially at high concentrations. Brine shrimp micro-well cytotoxicity assay was carried out for the methanol extract, and the commercial fungicide Bavistin FL. Cytotoxicity of the commercial fungicide was much higher (100% mortality at 250 ppm) compared to the methanol extract and methylripariochromene A. Thus, the aerial part of A. riparia can be commercially exploited as a natural alternative to commercially available fungicides for its ability to control banana anthracnose disease.