Background Irritable bowel syndrome (IBS) is one of the most common digestive diseases. The aim of this clinical trial was to determine the effectiveness of Achillea wilhelmsii C. Koch on the symptom severity and quality of life (QOL) in patients with IBS. Methods The patients were randomized into two groups of 45 each. The QOL and symptom severity of the patients were evaluated at baseline and at completion of the treatments by means of IBS-QOL and IBS severity index. Results The mean severity of clinical symptoms in the Achillea wilhelmsii C. Koch receiving groups before and after the treatment was 282.56 ± 103.57 and 178.06 ± 88.40, and in the placebo group was 265.93 ± 93.56 and 197.74 ± 106.26, respectively. The mean QOL in the Achillea wilhelmsii C. Koch receiving group before and after treatment was 51.49 ± 11.98 and 50.44 ± 13.39 and in the placebo group was 60.71 ± 11.97 and 58.39 ± 11.67, respectively. In both groups, there was a significant difference in the recovery rate in each group (p<0.05). However, the mean difference between the two groups before and after intervention was not significantly different (p>0.05). Also, no patient reported any adverse events during the trial. Although the symptom severity and QOL in both groups were improved compared to those before intervention, there was no significant difference between the two groups. Conclusion It is recommended to conduct future studies with larger sample size and longer treatment periods, and also investigate the efficacy on the IBS subtypes, separately.