In this study, the toxic effects of potassium bromate (KBrO3) were tested on Allium cepa L. meristematic cells. In order to determine the toxic effect and dose relationship, KBrO3 toxicity was investigated at doses of 25, 50, and 100 mg/L. The toxic effects were evaluated by using cytogenetic, biochemical, anatomical, and physiological parameters, and serious damages were observed depending on the dose. Significant reductions in germination percentage, weight gain, and radicle length were observed in all KBrO3-treated groups compared with the control. Mitotic activity decreased in meristematic cells after KBrO3 application. and mitotic index was decreased by 1.8 times in 100 mg/L KBrO3-treated group compared with the control group. The frequencies of micronucleus and chromosomal abnormalities tested as cytogenetic parameters were significantly higher in the group treated with 100 mg/L KBrO3 than those in the control group. Fragment and sticky chromosome were the most common types of chromosomal abnormalities. Lipid peroxidation measured in terms of MDA content increased with increasing doses of KBrO3. The activities of catalase and superoxide dismutase as antioxidant enzymes were importantly changed in KBrO3-treated groups. Anatomical changes such as cell deformation, substance accumulation, cell wall thickening, and flattened nucleus were determined after KBrO3 application, and it was observed that these changes reached a maximum level at 100 mg/L dose of KBrO3. As a result, KBrO3 treatments were been found to cause physiological, biochemical, cytogenetic, and anatomically toxic effects in meristematic cells of A. cepa, a eukaryotic model organism. The versatile toxicity induced by KBrO3 increased depending on the dose and reached a maximum level at 100 mg/L.