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Esophageal squamous cell carcinoma (ESCC) is a highly lethal malignancy characterized by significant radioresistance and poor prognosis. We previously reported that ribosomal S6 protein kinase 4 (RSK4) plays a pivotal role in promoting cancer stem cell (CSC) properties and radioresistance in ESCC. This study focuses on the regulation of post-translational modifications (PTMs) of RSK4 and their effects on CSC properties and radioresistance. We demonstrate that RSK4 stability and activity are tightly regulated by phosphorylation and O-GlcNAcylation. GSK3β phosphorylates RSK4 at Thr402/Ser406, promoting its degradation via the FBXW7-dependent proteasomal pathway. Additionally, O-GlcNAcylation of RSK4 at Thr405 by OGT inhibits GSK3β-mediated phosphorylation, stabilizing RSK4 and enhancing CSC properties and radioresistance. This antagonistic relationship between phosphorylation and O-GlcNAcylation highlights a novel regulatory mechanism of RSK4 in ESCC. Moreover, targeting RSK4 O-GlcNAcylation with OSMI-4 destabilizes RSK4 and sensitizes ESCC to radiotherapy in both patient-derived xenograft and organoid models. Collectively, this study provides critical insights into the molecular mechanisms underlying ESCC radioresistance and identifies RSK4 O-GlcNAcylation as a potential therapeutic target to improve radiotherapy efficacy and overcome treatment resistance.