The endoplasmic reticulum (ER) has a central role in processes essential for mounting effective and durable antitumour immunity; this includes regulating protein synthesis, folding, modification and trafficking in immune cells. However, the tumour microenvironment imposes hostile conditions that disrupt ER homeostasis in both malignant and infiltrating immune cells, leading to chronic activation of the unfolded protein response (UPR). Dysregulated ER stress responses have emerged as critical modulators of cancer progression and immune escape, influencing the initiation, development and maintenance of antitumour immunity. In this Review, we examine how tumour-induced ER stress reshapes the functional landscape of immune cells within the tumour microenvironment. We highlight recent discoveries demonstrating how ER stress curtails endogenous antitumour immunity and reduces the efficacy of immunotherapies. Furthermore, we underscore novel therapeutic strategies targeting ER stress sensors or UPR components to restore immune function and enhance cancer immunotherapy outcomes. Together, this provides a comprehensive overview of the interplay between ER stress responses and antitumour immunity, emphasizing the potential of UPR-targeted interventions to improve immune control of cancer.