Radiotherapy is a crucial treatment for patients undergoing breast cancer surgery, especially for those with locally advanced stage of breast cancers. However, low radiosensitivity significantly limits the efficacy of radiotherapy. Epigenetic regulation plays a critical role in cancer treatment, and histone H3 lysine acetylation (H3ac) affects the redox balance during homeostasis. Bromodomain-containing protein 3 (BRD3) is an H3ac reader, and its overexpression is associated with invasion, metastasis, and chemoresistance. However, its correlation with radiosensitivity remains unclear. Here, we reported that radiotherapy induced a significant increase in BRD3 level, whereas deacetylase inhibitors reduced BRD3 expression. ChIP-seq results showed that BRD3 binds to the RAD51 promoter region, which is involved in radioresistance, inhibiting RAD51 transcription and translation. Furthermore, the inhibition of RAD51 was mainly observed in the cytoplasm, revealing a novel mechanism in which RAD51 forms a complex with GPX4 and FTH1 to synergistically regulate ROS production. These findings suggest that H3ac regulates BRD3 expression, promoting ROS accumulation through the RAD51-mediated ferroptosis signaling pathway, reversing radioresistance and enhancing radiosensitivity.