Abstract Introduction Peroxiredoxin-1 (PRDX1) is a multifunctional protein, acting as a hydrogen peroxide (H 2 O 2 ) scavenger, molecular chaperone and immune modulator. Although differential PRDX1 expression has been described in many tumors, the potential role of PRDX1 in breast cancer remains highly ambiguous. Using a comprehensive antibody-based proteomics approach, we interrogated PRDX1 protein as a putative biomarker in estrogen receptor (ER)-positive breast cancer. Methods An anti-PRDX1 antibody was validated in breast cancer cell lines using immunoblotting, immunohistochemistry and reverse phase protein array (RPPA) technology. PRDX1 protein expression was evaluated in two independent breast cancer cohorts, represented on a screening RPPA (n = 712) and a validation tissue microarray (n = 498). In vitro assays were performed exploring the functional contribution of PRDX1, with oxidative stress conditions mimicked via treatment with H 2 O 2 , peroxynitrite, or adenanthin, a PRDX1/2 inhibitor. Results In ER-positive cases, high PRDX1 protein expression is a biomarker of improved prognosis across both cohorts. In the validation cohort, high PRDX1 expression was an independent predictor of improved relapse-free survival (hazard ratio (HR) = 0.62, 95% confidence interval (CI) = 0.40 to 0.96, P  = 0.032), breast cancer-specific survival (HR = 0.44, 95% CI = 0.24 to 0.79, P  = 0.006) and overall survival (HR = 0.61, 95% CI = 0.44 to 0.85, P  = 0.004). RPPA screening of cancer signaling proteins showed that ERα protein was upregulated in PRDX1 high tumors. Exogenous H 2 O 2 treatment decreased ERα protein levels in ER-positive cells. PRDX1 knockdown further sensitized cells to H 2 O 2 - and peroxynitrite-mediated effects, whilst PRDX1 overexpression protected against this response. Inhibition of PRDX1/2 antioxidant activity with adenanthin dramatically reduced ERα levels in breast cancer cells. Conclusions PRDX1 is shown to be an independent predictor of improved outcomes in .