Small cell lung cancer (SCLC) is a highly aggressive neuroendocrine malignancy with few effective targeted therapies. As hypoxia is a central feature of solid tumours and contributes to resistance mechanisms, identifying therapeutic vulnerabilities linked to hypoxia-responsive pathways is crucial. The gastrin-releasing peptide receptor (GRPR) is commonly elevated in SCLC, yet its regulation under hypoxia has not been extensively characterised. In this study, we examined the effect of hypoxia on GRPR expression and evaluated the therapeutic potential of a butylated neuropeptide antagonist (Bu peptide) specifically designed to inhibit GRPR-mediated signalling. Exposure of DMS79 and COR-L24 SCLC cells to hypoxia led to a pronounced increase in GRPR expression. This was validated using Western blot analysis and BBN-FITC binding assays, confirming hypoxia-driven enhancement of receptor availability at the cell surface. The Bu peptide, synthesised through an optimised Boc-based solid-phase peptide synthesis strategy, demonstrated strong cytotoxic activity in both cell lines. Its potency was more pronounced under hypoxic conditions, aligning with elevated GRPR expression. Mechanistic studies showed that Bu peptide treatment effectively suppressed phosphorylation of AKT and ERK, two key downstream mediators of GRPR signalling involved in survival and proliferation. In addition, Bu peptide triggered significant apoptotic activation, as evidenced by increased caspase-3/7 activity, with stronger effects observed under hypoxia. Overall, our findings highlight GRPR as a hypoxia-inducible therapeutic target in SCLC and demonstrate that the Bu peptide selectively inhibits GRPR-driven pathways while inducing apoptosis in hypoxic tumour cells. These results underscore the translational relevance of GRPR antagonists and support further development of tailored peptide-based therapies for hypoxia-adaptive cancers.