Although Wnt/β-catenin pathway activation has been implicated in mouse models of breast cancer, there is contradictory evidence regarding its importance in human breast cancer. In this study, invasive and in situ breast cancer tissue microarrays containing luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)⁺/ER⁻ and basal-like breast cancers were analyzed for β-catenin subcellular localization. We demonstrate that nuclear and cytosolic accumulation of β-catenin, a read-out of Wnt pathway activation, was enriched in basal-like breast cancers. In contrast, membrane-associated β-catenin was observed in all breast cancer subtypes, and its expression decreased with tumor progression. Moreover, nuclear and cytosolic localization of β-catenin was associated with other markers of the basal-like phenotype, including nuclear hormone receptor and HER2 negativity, cytokeratin 5/6 and vimentin expression, and stem cell enrichment. Importantly, this subcellular localization of β-catenin was associated with a poor outcome and is more frequently observed in tumors from black patients. In addition, β-catenin accumulation was more often observed in basal-like in situ carcinomas than other in situ subtypes, suggesting that activation of this pathway might be an early event in basal-like tumor development. Collectively, these data indicate that Wnt/β-catenin activation is an important feature of basal-like breast cancers and is predictive of worse overall survival, suggesting that it may be an attractive pharmacological target for this aggressive breast cancer subtype.