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Combination therapies using checkpoint inhibitors with immunostimulatory agonists have attracted great attention due to their synergistic therapeutic effects for cancer treatment. However, such combination immunotherapies require specific timing of doses to show sufficient antitumor efficacy. Sequential treatment usually requires multiple administrations of the individual drugs at specific time points, thus increasing the complexity of the drug regimen and compromising patient compliance. Here, we introduce an injectable porous silicon microparticle (pSiMP) for combination cancer immunotherapy where its multilayered nanopore structure was electrochemically programmed to achieve release of three distinct immunomodulatory drugs in the right sequence at the desired time. We find the optimal sequential treatment timeline of stimulator of interferon genes (STING) agonist, anti-OX40 antibody (aOX40), and anti-PD-1 antibody (aPD-1) for immunosuppressive tumors. We show that a single intratumoral injection of a cocktail of release-programmed pSiMPs coloaded with each antibody and a STING agonist significantly suppresses the tumor growth compared to conventional treatment involving sequential bolus injections, or an injection of pSiMPs configured to release all drugs at the same time, with no delay. With the timely release of immunomodulatory drugs, the programmable pSiMPs offer an effective treatment strategy for combination immunotherapy.