Background: Ginsenoside Rg5 possesses potent anti-oxidative, anti-inflammatory, and cytoprotective properties. This study explored the protective effects of ginsenoside Rg5 on radiation-induced pulmonary microvascular endothelial cells (PMECs) injury and the associated molecular mechanisms. Materials and methods: C57BL/6 mice were used for in vivo studies and primary human PMECs (PPMECs) were utilized as in vitro models. Mice with or without ginsenoside Rg5 pretreatment were irradiated by varying doses. Lung tissues were analyzed for histopathological changes and the expression of endothelial markers. In vitro, PPMECs were irradiated with or without ginsenoside Rg5 pretreatment and analyzed for apoptosis, oxidative stress, mitochondrial function, and endothelial barrier integrity. Results: Ginsenoside Rg5 pretreatment attenuated radiation-induced acute lung damage, preserved endothelial cell junction integrity, and maintained endothelial barrier function in vivo. In vitro, ginsenoside Rg5 significantly reduced IR-induced oxidative stress, apoptosis, and mitochondrial dysfunction in PPMECs. Ginsenoside Rg5 suppressed radiation-induced Mfn2 acetylation and proteasomal degradation via Sirt1-mediated deacetylation, thereby preserving mitochondrial dynamics and integrity. The protective effects of ginsenoside Rg5 on the integrity of mitochondrial and endothelial tight junction proteins and barrier function were also Sirt1-dependent. Conclusions: Ginsenoside Rg5 exerts a protective effect against radiation-induced endothelial injury by modulating mitochondrial dynamics and function, as well as maintaining endothelial barrier integrity, in a Sirt1dependent manner.