The present study evaluated the genotoxic effect of aqueous extract of C. sarcomedium Meve and Liede and its protective role in chromosomal aberrations induced by 2% hydrogen peroxide (H2O2) on Allium cepa root meristem. Our results showed that the C. sarcomedium extract induced major chromosomal aberrations, viz., nuclear lesions, chromosome breaks, chromosome laggards, vagrance, stickiness, and bridges. These are irreversible in nature and indicate that the plant is a potent clastogen. Total chromosome aberrations were more frequent with increase in exposure time and concentrations of plant extract. For antigenotoxic screening, three different modes of treatment were used: pretreatment, posttreatment, and simultaneous treatment. The significant induction of a wide spectrum of chromosomal aberrations and the reduction in mitotic index effected by the aqueous extract reinforced the genotoxic potential of the plant. The modulatory effect of plant extract was expressed in terms of inhibition percentage and had an excellent effect in reversing chromosomal aberrations induced by H2O2. Dose-dependent results suggest the genotoxic and antigenotoxic effect of C. sarcomedium on onion root tips. These findings offer a useful basis for further investigations on C. sarcomedium, a shrub with a broad array of prospective therapeutic activities.