Immunotherapy has made great strides in improving clinical outcomes in cancer treatment. However, few patients exhibit adequate response rates for key outcome measures and desired long-term responses, and they often suffer systemic side effects due to the dynamic nature of the immune system. This has motivated a search for alternative strategies to improve unsatisfactory immunotherapeutic outcomes. In recent years, biomaterial-assisted immunotherapy has shown promise in cancer treatment with improved therapeutic efficacy and reduced side effects. These biomaterials have illuminated fundamental mechanisms underlying the immunoediting process, while greatly improving the efficacy of chimeric antigen receptor (CAR) T-cell therapy, cancer vaccine therapy, and immune checkpoint blockade therapy. This Minireview discusses recent advances in engineered biomaterials that address limitations associated with conventional cancer immunotherapies.