Background and Objective: Metformin pretreatment might have neuroprotective effects. We aimed to determine the therapeutic effects of the antidiabetic medication metformin on ischemic stroke severity and discharge outcomes. Methods: We analyzed data on 1303 ischemic stroke patients who were on antidiabetic medications from the Massachusetts General Hospital (MGH) Advanced Comprehensive Stroke Center dataset (n = 8943, 2012-2022). We applied propensity score matching (PSM) and inverse probability of treatment weighting (IPTW) analyses to investigate the effect of current usage of metformin (versus alternate antidiabetic treatment) on acute stroke clinical severity and discharge outcomes. Results: Of the 1303 patients who were on antidiabetic medications at the time of stroke admission, 730 (56%) were taking metformin. Metformin users were younger and more frequently had hypertension, whereas less frequently had prior CAD, AFib, and chronic kidney disease. The clinical features and laboratory values of the two groups were evenly distributed after PSM. Metformin-treated patients had statistically significant lower stroke severity on admission [National Institutes of Health Stroke Scale (NIHSS) (median, interquartile range) 3.0 (1.0-8.0) vs. 4.0 (2.0-11.3), p = 0.011], better functional independence at discharge (modified Rankin scale score 0-2, 36.3% vs. 25.4%, p < 0.001) and less in-hospital mortality (4.5% vs. 11.3%, p = 0.018). IPTW analysis results were consistent with PSM results. Conclusions: Among diabetic patients with acute ischemic stroke, metformin appears to confer neuroprotection. Our results extend previous findings to the general stroke population. Stroke patients with diabetes mellitus who were treated with metformin prior to stroke, even when combined with additional antidiabetic medications, experienced less severe strokes upon admission and had better functional outcomes during hospitalization.