Genotoxic and mutagenic effects of aqueous extract from aerial parts of Achillea teretifolia (At-ext) were investigated using both Allium cepa root meristematic cells and a short-term mutation assay in Salmonella typhimurium with TA98 and TA100 strains in the presence or absence of S9 mix, respectively. In the Allium tests, EC50 value was determined as 50 g/L, and 0.5 × EC50, EC50, and 2 × EC50 concentrations of At-ext were applied to onion tuber roots. Methyl methane sulfonate (MMS, 10 ppm) and distilled water were used as positive and negative controls, respectively. The mitotic index (MI) increased in all concentrations in comparison to control at each exposure time except 25 and 50 g/L at 12 h. While stickiness, chromosome laggards, bridges, and disturbed anaphase-telophase were observed in anaphase-telophase cells, pro-metaphase, c-metaphase, polyploidy, and binuclear cells were observed in other cells. It was determined that At-ext have no mutagenic effect on S. typhimurium TA98 and TA100 strains in the presence or absence of S9 mix. The results were statistically analyzed in SPSS using Duncan’s multiple range test. These results indicate that At-ext has genotoxic activity in A. cepa root meristematic cells.