The monitoring of landfill leachate in the Brazilian semiarid region has traditionally been based on physicochemical parameters which, although essential, were insufficient to determine effluent toxicity and to predict complex biological risks. The objective of this study was to evaluate leachate toxicity through bioassays using Allium cepa (A. cepa). The methodology comprised physicochemical characterization (organic and inorganic parameters and metals) and toxicity bioassays with A. cepa using an in natura leachate sample. The phytotoxicity, genotoxicity, and mutagenicity of the effluent were assessed at different concentrations (1, 5, 10, 15, and 100%), employing control samples (positive and negative) for comparison. The results revealed an alkaline effluent with high organic loads and the presence of metals such as chromium. Biologically, the 1% concentration induced a stimulatory effect on root growth, whereas concentrations above 15% were highly phytotoxic. However, the key contribution lies in the finding that genotoxicity was observed at all concentrations, including those without apparent phytotoxic effects, and mutagenicity was detected from 10% onward. These findings establish a new analytical perspective by demonstrating that the absence of acute phytotoxicity does not ensure cellular integrity, revealing a disconnect between positive root growth and the occurrence of underlying genetic damage induced by the leachate. The climatic dynamics of the semiarid region exacerbate leachate toxicity, highlighting that the inclusion of genetic indicators in regulatory frameworks to address gaps that isolated physicochemical analyses cannot predict is urgent.