Cancer immunotherapy can successfully promote long-term anticancer immune responses, although there is still only a limited number of patients who benefit from such treatment, and it can sometimes have severe treatment-associated adverse events. Compared with systemic immunomodulation, local immunomodulation may enable more effective treatment at lower doses and, at the same time, prevent systemic toxicity. Local delivery of engineered three-dimensional scaffolds may fulfil this role by acting as synthetic immune niches that boost anticancer immunity. In this Opinion article, we highlight the potential of scaffold-based adoptive cell transfer and scaffold-based cancer vaccines that, although applied locally, can promote systemic antitumour immunity. Furthermore, we discuss how scaffold-based cancer immunotherapy may contribute to the development of the next generation of cancer treatments.