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Radiotherapy remains an irreplaceable treatment modality for breast cancer (BC). Calmodulin‑binding Transcription Activator 1 (CAMTA1) has been implicated in tumor progression; however, its role in BC is unclear. The present study aimed to elucidate the mechanistic function of CAMTA1 in BC. RNA sequencing was performed on RAW264.7 macrophages co‑cultured with 4T1 cells and subjected to X‑ray irradiation. In vitro, THP‑1 cells were co‑cultured with MDA‑MB‑231 cells under hypoxic conditions. Exosome morphology was observed under transmission electron microscopy and PKH67 staining was used to trace exosome uptake. Flow cytometry was used to detect CD163 expression while ELISA measured the levels of IL‑10 and IL‑12. Reverse transcription‑quantitative (RT‑q) PCR and immunoblotting analysis were used to detect the expressions of neuregulin 1 (NRG1), CAMTA1 and hypoxia‑inducible factor‑1α. Cell apoptosis, cell cycle distribution, cell viability and proliferation were evaluated using flow cytometry, MTT assay and colony formation assay. In vivo, transfected MDA‑MB‑231 cells were injected into BALB/c nude mice combined with radiotherapy and exosome injection. Histopathological changes in tumor tissues were examined using H&E staining. Immunohistochemistry analysis was performed to assess the expressions of NRG1, Caspase‑3 and CD163. RNA sequencing, RT‑qPCR and immunoblotting analysis revealed that NRG1 expression was markedly increased in RAW264.7 macrophages co‑cultured with 4T1 cells. NRG1 was found to be involved in M2 polarization induced by hypoxia‑treated MDA‑MB‑231 cells, which in turn promoted radio‑resistance. CAMTA1 expression was highly expressed in exosomes derived from hypoxic MDA‑MB‑231 cells and exosomal CAMTA1 promoted the M2 polarization of THP‑1 macrophages. In vivo, CAMTA1 overexpression greatly enhanced tumor growth, increased NRG1 expression, inhibited cell apoptosis and promoted M2 polarization of macrophages in tumor tissue. MDA‑MB‑231 cells were found to deliver CAMTA1 to macrophages via exosomes, leading to upregulation of NRG1 and induction of M2 polarization, thereby enhancing BC cells resistance to radiotherapy. These findings provided novel insights into the mechanisms underlying radio‑resistance in BC and identify exosomal CAMTA1 as a potential therapeutic target.