Background: Eosinophilic gastritis (EG) is a clinicopathologic disorder with marked gastric eosinophilia and clinical symptoms. There is an unmet need among patients with EG for more precise diagnostic tools. Objective: We aimed to develop tissue-and blood-based diagnostic platforms for EG. Methods: Patients with EG and control subjects without EG were enrolled across 9 Consortium of Eosinophilic Gastrointestinal Disease Researchers-associated sites. An EG Diagnostic Panel (EGDP; gastric transcript subset) and EG blood biomarker panel (protein multiplex array) were analyzed. EGDP 18 scores were derived from the expression of 18 highly dysregulated genes, and blood EG scores were derived from dysregulated cytokine/chemokine levels. Results: Gastric biopsy specimens and blood samples from 185 subjects (patients with EG, n 5 74; control subjects without EG, n 5 111) were analyzed. The EGDP (1) identified patients with active EG (P < .0001, area under the curve > _ 0.95), (2) effectively monitored disease activity in longitudinal samples (P 5 .0078), (3) highly correlated in same-patient samples (antrum vs body, r 5 0.85, P < .0001), and (4) inversely correlated with gastric peak eosinophil levels (r 5 20.83, P < .0001), periglandular circumferential collars (r 5 20.73, P < .0001), and endoscopic nodularity (r 5 20.45, P < .0001). For blood-based platforms, eotaxin-3, thymus and activation-regulated chemokine, IL-5, and thymic stromal lymphopoietin levels were significantly increased. Blood EG scores (1) distinguished patients with EG from control subjects without EG (P < .0001, area under the curve > _ 0.91), (2) correlated with gastric eosinophil levels