The H5N1 avian influenza virus clade 2.3.4.4b outbreak represents a major pandemic threat for humans, with some reported cases of severe and fatal respiratory illness. A key unanswered question is the pathogenesis of severe H5N1 disease following respiratory infection. In this study, we explored mechanisms of pathogenesis of severe H5N1 disease in cynomolgus and rhesus macaques following infection with the H5N1 isolate A/Texas/37/2024 (huTX37-H5N1). Cynomolgus macaques developed severe pneumonia that was lethal in 100% of macaques by 7 days post-infection. By contrast, rhesus macaques demonstrated dose-dependent mortality, and surviving animals showed protective immunity against high-dose re-challenge. A multi-omics analysis demonstrated that H5N1 infection was characterized by robust induction of proinflammatory cytokines, innate immune cells, complement, coagulation, apoptosis, and immune exhaustion pathways. Taken together, our data indicate inflammation and immune dysregulation as key mechanisms of H5N1 pathogenesis in nonhuman primates.