Introduction: Aluminum phosphide (AlP) ingestion for self-harm is associated with a high case-fatality rate (CFR) in low- and middle-income countries. A reliable and accurate prognostic scoring tool is required for appropriate triaging, to guide clinical decision-making, and to evaluate the efficacy of therapeutic interventions for the patients with AlP toxicity. Materials and methods: We performed a prospective cohort study in a tertiary care hospital in north India in patients aged 15 years and over with acute AlP poisoning, investigating the parameters associated with CFR, and developing a reliable and simple prediction score. Results: The CFR was 51% in this cohort of 105 patients. Three parameters—pH <7.25, score on Glasgow coma scale (GCS) <13, and systolic blood pressure (SBP) <87 mm Hg were most robust predictors of CFR (odds ratio; 12.614, 18.621, and 17.600, respectively; area under the receiver operating characteristic curve—0.808, 0.796, and 0.776, respectively). Based on these parameters (with 1 point to each), a prognostic score was developed, ranging from 0 to 3 points. A total score of 3 had a 98.2% specificity and a positive predictive value of 96.4%, whereas a score ≤1 had a 100% sensitivity and 100% negative predictive value. Conclusion: A scoring system based on low pH (P), low GCS score (G), and impaired or low SBP (I) (“PGI” score) may provide a simplified predictive model for mortality in AlP poisoning.