Triple-negative breast cancer (TNBC) is characterised by its aggressiveness and resistance to chemotherapy, demanding the development of effective strategies against its unique characteristics. Derived from lapacho tree bark, β-lapachone (β-LP) selectively targets cancer cells with elevated levels of the detoxifying enzyme NQO1. Hydroxytyrosol (HT) is a phenolic compound derived from olive trees with important anticancer properties that include the inhibition of cancer stem cells (CSCs) and metastatic features in TNBC, as well as relevant antioxidant activities by mechanisms such as the induction of NQO1. We aimed to study whether these compounds could have synergistic anticancer activity in TNBC cells and the possible role of NQO1. For this pourpose, we assessed the impact of β-LP (0.5 or 1.5 μM) and HT (50 and 100 μM) on five TNBC cell lines. We demonstrated that the combination of β-LP and HT exhibits anti-proliferative, pro-apoptotic, and cell cycle arrest effects in several TNBC cells, including docetaxel-resistant TNBC cells. Additionally, it effectively inhibits the self-renewal and clonogenicity of CSCs, modifying their aggressive phenotype. However, the notable impact of the β-LP-HT combination does not appear to be solely associated with the levels of the NQO1 protein and ROS. RNA-Seq analysis revealed that the combination's anticancer activity is linked to a strong induction of endoplasmic reticulum stress and apoptosis through the unfolded protein response. In conclusion, in this study, we demonstrated how the combination of β-LP and HT could offer an affordable, safe, and effective approach against TNBC.