Diabetic ketoacidosis (DKA) is a metabolic complication of diabetes mellitus that takes a lethal course if untreated. In this way relevant to forensic medicine, secure diagnosis of DKA usually involves the evidence of elevated levels of glucose and the ketone bodies acetone, acetoacetate, and β-hydroxybutyrate in corpse fluids. We conducted a postmortem hydrogen proton magnetic resonance spectroscopy ($^1$H-MRS) in a case of lethal DKA. Distinctive resonances of all three ketone bodies as well as glucose were visible in spectra of cerebrospinal fluid, vitreous humor, and white matter. Estimated concentrations of ketone bodies and glucose supported the findings both of autopsy and biochemical analysis. Advantages of human postmortem $^1$H-MRS are the lack of movement and flow artifacts as well as lesser limitations of scan duration. Postmortem $^1$H-MRS is able to non-invasively measure concentrations of glucose and ketone bodies in small volumes of various regions of the brain. It may thus become a diagnostic tool for forensic investigations by quick determination of pathological metabolite concentrations in addition to conventional autopsy.