Natural systems use metal ions to form ordered structures that regulate biological processes, inspiring the rational design of nanotherapeutics. The cyclic guanosine monophosphate-adenosine monophosphate synthase-stimulator of interferon genes (cGAS-STING) pathway drives antitumor immunity but has been difficult to activate systemically owing to poor pharmacology and toxicity. Here, we report CRYSTAL, a structurally ordered intermetallic nanoparticle for potent systemic STING activation. CRYSTAL self-assembles from manganese ions intercalated with cyclic dinucleotides, enabling precise structural control. At an ultralow intravenous dose (0.003 milligrams per kilogram), CRYSTAL activated STING in mice, dogs, and nonhuman primates without cytokine release syndrome. CRYSTAL induced robust tumor regression in advanced murine and rabbit models, remodeled immunosuppressive environments, and promoted host STING-dependent CD8+ T cell priming. CRYSTAL activated interferon responses in human head and neck squamous cell carcinoma biopsies, underscoring its translational potential for cancer immunotherapy.