Metformin is the most widely used drug to treat type 2 diabetes and its mitochondrial activity is through activation of adenosine monophosphate-activated protein kinase (AMPK). AMPK plays a dual regulatory role in mito-morphosis, controlling the phosphorylation and activation of dynamin-related protein 1 (DRP1) and mitofusin 2 (MFN2). The aim of this study was to investigate whether metformin could reduce early brain injury (EBI) after subarachnoid hemorrhage (SAH) by activating mitophagy and improving mitochondrial morphology through AMPK. This study used 308 male Sprague-Dawley rats. First, different metformin doses were injected intraperitoneally 30 min post-SAH. The dose that did not significantly alter blood glucose in the rats was selected for subsequent experiments. Before or after sacrificing rats, neurological function, brain water content, and blood-brain barrier (BBB) permeability were measured in each group. Transmission electron microscopy was used to observe the level of mitophagy and mito-morphology in each group. The expression of mitophagic and apoptotic proteins were investigated by immunofluorescence and western blot. Metformin at 20 mg/kg improved neurological function and attenuated brain edema and the disruption of BBB permeability 24 h after SAH. Metformin treatment after SAH promoted mitophagy in an AMPK-dependent manner. In addition to the effects on mitophagy, we also found that metformin alleviated oxidative stress and apoptosis after SAH in an AMPK-dependent manner. Lastly, metformin restored homeostasis between mitochondrial fusion and fission. Metformin attenuated EBI after SAH in rats through AMPK-dependent signaling. These protective effects might be achieved by regulating mitochondrial morphology and promoting mitophagy.Copyright © 2022 Elsevier Inc. All rights reserved.