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Abstract Background Predictive biomarkers for oesophageal squamous cell carcinoma (ESCC) immunotherapy are lacking, and immunotherapy resistance remains to be addressed. The role of long noncoding RNA (lncRNA) in ESCC immune escape and immunotherapy resistance remains to be elucidated. Methods The tumour‐associated macrophage‐upregulated lncRNAs and the exosomal lncRNAs highly expressed in ESCC immunotherapy nonresponders were identified by lncRNA sequencing and polymerase chain reaction assays. CRISPR‐Cas9 was used to explore the functional roles of the lncRNA. RNA pull‐down, MS2‐tagged RNA affinity purification (MS2‐TRAP) and RNA‐binding protein immunoprecipitation (RIP) were performed to identify lncRNA‐associated proteins and related mechanisms. In vivo, the humanized PBMC (hu‐PBMC) mouse model was established to assess the therapeutic responses of specific lncRNA inhibitors and their combination with programmed cell death protein 1 (PD‐1) monoclonal antibody (mAb). Single‐cell sequencing, flow cytometry, and multiplex fluorescent immunohistochemistry were used to analyze immune cells infiltrating the tumour microenvironment. Results We identified a lncRNA that is involved in tumour immune evasion and immunotherapy resistance. High LINC02096 (RIME) expression in plasma exosomes correlates with a reduced response to PD‐1 mAb treatment and poor prognosis. Mechanistically, RIME binds to mixed lineage leukaemia protein‐1 (MLL1) and prevents ankyrin repeat and SOCS box containing 2 (ASB2)‐mediated MLL1 ubiquitination, improving the stability of MLL1. RIME‐MLL1 increases H3K4me3 levels in the promoter regions of programmed death‐ligand 1 (PD‐L1) and indoleamine 2,3‐dioxygenase 1 (IDO‐1), constitutively increasing the expression of PD‐L1/IDO‐1 in tumour cells and inhibiting CD8+ T cells infiltration and activation. RIME depletion in huPBMC‐NOG mice significantly represses tumour development and improves the effectiveness of PD‐1 mAb treatment by activating T‐cell‐mediated antitumour immunity. Conclusions This study reveals that the RIME‐MLL1‐H3K4me3 axis plays a critical role in tumour immunosuppression. Moreover, RIME appears to be a potential prognostic biomarker for immunotherapy and developing drugs that target RIME may be a new therapeutic strategy that overcomes immunotherapy resistance and benefits patients with ESCC.