The coactivator-associated arginine methyltransferase-1 (CARM1) is implicated in regulation of oestrogen receptor (ER) α-mediated gene pathways in response to ER activation. It plays an important role in breast cancer growth by regulating the E2F1 expression suggesting that CARM1 could be a target in the subclassification of oestrogen-dependent breast cancer. This study aims to investigate the clinical and biological importance of CARM1 protein expression in a large (1,130 patients), well-characterised and annotated series of invasive breast cancers using tissue microarrays and immunohistochemistry. In the whole series, increased CARM1 expression is correlated with features associated with aggressive behaviour such as young age, premenopausal status, large tumour size and high tumour grade. There is a positive correlation between CARM1 expression and biomarkers associated with non-luminal phenotype and poor prognosis such as HER2, basal cytokeratins, EGFR, p53 and the proliferation markers Ki67, TK1, CD71 and Cyclin E. Negative associations with the luminal-associated markers including steroid receptors and luminal cytokeratins are found. Similar associations are identified in the ER-positive/luminal subgroup (n = 767). Outcome analyses indicate that CARM1 expression is an independent predictor of shorter breast cancer-specific survival and disease-free interval in the whole series and in the ER-positive subgroup. CARM1 shows an oncogenic effect in breast cancer and its expression is associated with poor prognosis. CARM1 could be a potential marker of luminal class subclassification and for target therapy, particularly in the ER-positive luminal-like subgroup.