When properly deployed, the immune system can render deadly pathogens harmless, eradicate metastatic cancers, and provide long-lasting protection from diverse diseases. However, realizing these remarkable capabilities is inherently risky, as disruption to immune homeostasis can lead to dangerous complications and autoimmune disorders. While current research is continuously expanding the arsenal of potent immunotherapeutics, there is a technological gap when it comes to controlling when, where, and how long these drugs act on the body. Here, we explored the ability of a slow-releasing injectable hydrogel depot to reduce the problematic dose-limiting toxicities of immunostimulatory CD40 agonist antibodies (CD40a) while maintaining their potent anti-cancer efficacy. We previously described a polymer-nanoparticle (PNP) hydrogel system that is biocompatible, long lasting, and injectable, traits that we hypothesized would improve locoregional delivery of the CD40a immunotherapy. Using PET imaging, we found that hydrogels significantly improve CD40a pharmacokinetics by redistributing drug exposure to the tumor and the tumor draining lymph node (TdLN). Consistent with this altered biodistribution, hydrogel delivery significantly reduced weight loss, hepatotoxicity, and cytokine storm associated with treatment. Moreover, CD40a-loaded hydrogels were able to mediate improved local cytokine induction in the TdLN and improve treatment efficacy in both mono- and combination therapy settings in the B16F10 melanoma model. These results suggest that PNP hydrogels are a facile, drug-agnostic method to ameliorate immune-related adverse effects and explore locoregional delivery of immunostimulatory drugs.