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Introduction T-cell acute lymphoblastic leukaemia (T-ALL) is an aggressive cancer arising from T-cell progenitors. Although current treatments, including chemotherapy and glucocorticoids, have significantly improved survival, T-ALL remains a fatal disease and new treatment options are needed. Since more than 60% of T-ALL cases bear oncogenic NOTCH1 mutations, inhibitors of NOTCH1 signalling are being actively investigated for the treatment of T-ALL. However, they have limited efficacy in tumours due to dose-limiting toxicities. Recent evidence indicates that γ-secretase activity not only occurs at the cell surface, but also in the acidic environment of lysosomes and endosomes. In addition, endosomes have been shown to play a key role in the NOTCH signal relay. Therefore, we hypothesised that chloroquine (CQ), a clinically used antimalarial drug that inhibits lysosomal function and autophagy, may inhibit T-ALL growth and survival. Material and methods T-ALL cell lines were treated with γ-secretase inhibitor (GSI), CQ or a combination treatment. Cell viability was determined by trypan blue exclusion. Propidium Iodide (PI) staining was combined with Annexin V staining or Edu incorporation to measure apoptosis and perform cell cycle analysis, respectively. Flow cytometry was used to determine cytoplasmic ROS levels and yH2AX expression. To address the effect of CQ on NOTCH1 signalling, cleaved NOTCH1 was assessed by immunoblotting, NOTCH1 target gene expression by qPCR and NOTCH1 expression at the cell surface by flow cytometry. In vivo T-ALL xenografts were grown and treated with different doses of GSI, CQ or a combination treatment. Tumour growth and gastro-intestinal toxicity were assessed. Results and discussions CQ decreases T-ALL cell viability and proliferation and reduces GSI sensitive T-ALL xenograft growth, which was not observed in GSI-resistant T-ALL cell lines or xenografts. Mechanistically, CQ impairs the redox balance and induces DNA damage with a subsequent DNA damage response. In addition, we observed that CQ interferes with intracellular trafficking and processing of oncogenic NOTCH1. Interestingly, we show for the first time that the addition of CQ to γ-secretase inhibition has an additive therapeutic effect on T-ALL and reduces the concentration of GSI required to obtain tumour control. Conclusion Overall, our results suggest that CQ may be a promising alternative as a repurposed drug in the treatment of T-ALL, either as a single treatment or in combination with NOTCH1 inhibition, increasing the therapeutic ratio.