One aspect relates to a biomarker for estimating prognosis in cancer patients and a use of same. According to one aspect, it was confirmed that it was possible to estimate prognosis in cancer patients, in particular patients with triple negative breast cancer, by checking mutations in glutathione peroxidase 4 (GPX4) gene or an expression product of the gene, thus demonstrating that the GPX4 gene can be utilized as a biomarker. Furthermore, since the biomarker can be used to improve prognosis of a cancer patient or to prevent or treat cancer, the biomarker may be utilized for various uses.