Genotoxicity studies are important in the assessment of landfill leachate toxicity. This study investigated the genotoxic potential of Olusosun landfill leachate (OLL) by assessing its effect on the formation of micronuclei in rat bone marrow after sub-acute and sub-chronic exposures; the genotoxic potential of the leachate was also assessed using Allium cepa chromosome aberration test. For the sub-acute and sub-chronic studies, rats were exposed to 0% - 100% OLL for 14 days and 90 days, respectively. Bone marrow cells of the rats were then examined for micronuclei formation. For the Allium cepa test, root growth inhibition, and induction of mitotic and chromosomal aberrations were observed in Allium cepa exposed to OLL. The leachate was also analyzed for some physicochemical parameters and heavy metals. The results revealed that there was a significant (p < 0.05) increase in the number of micronucleated polychromatic erythrocytes in bone marrow cells of rats following sub-chronic exposure to 10%, 50%, 75% and 100% OLL compared with control while micronuclei formation was only observed in rats exposed to 75% and 100% OLL sub-acutely. The leachate inhibited root growth, caused various morphological deformities and induced chromosomal aberrations in Allium cepa. The results of this study show that Olusosun landfill leachate is genotoxic as the leachate induced formation of micronuclei in bone marrow of rats and also significantly induced chromosomal aberrations in Allium cepa cells. This is of great concern as the leachate-contaminated ground waters could increase the risk of cancer and other genetic diseases in exposed individuals.