Geranium species are utilized as a folk medicine to treat diabetes mellitus in Turkey, Morocco, Algeria, Northern Peru, India.This study aimed to investigate the hypoglycemic activity of Geranium subcaulescens root (GSR) extract using in vitro, in vivo, in silico methods. It was planned to identify the compound(s) responsible for the activity through activity-directed isolation studies from GSR methanol extract and to standardize the extract based on the major compound using High-Performance Liquid Chromatography. The effects of GSR extract and isolated compounds on α-glucosidase, α-amylase, pancreatic lipase, pancreatic cholesterol esterase enzymes were investigated. Four models (normoglycemic model, oral glucose tolerance test, acute, subacute experiments in streptozotocin (STZ)-induced diabetic animals) were used for in vivo studies. The interactions of compounds with α-glucosidase α-amylase enzymes were investigated by molecular docking. The methanol extract strongly inhibited α-glucosidase (IC50 8.68 ± 0.11μg/ml), α-amylase enzymes (IC50 6.77 ± 1.58μg/ml) caused significant hypoglycemic effects in STZ diabetic rats. 3,3′ di-O-methylellagic acid 4′-O-α-rhamnopyranoside, 3,3′,4 tri-O-methylellagic acid 4′-O-α-rhamnopyranoside, geraniin, geranin A, ellagic acid 3-O-α-rhamnopyranoside were isolated. Geraniin was found to have inhibitory activity on both α-glucosidase (IC50 4.512 ± 0.200μg/ml) α-amylase (IC50 2.205 ± 1.500) enzymes and to cause significant hypoglycemic activity (31.74 % at a dose of 50 mg/kg and by 55.41 % at a dose of 75 mg/kg) in STZ-diabetic animals. In silico studies, it was exhibited that geraniin has strong interactions with the relevant enzymes. The findings support the folk medicinal use of Geranium species in treating diabetes. In vitro, in vivo, and in silico study results predicted that geraniin (75.54 mg/g plant), as the major compound, is responsible for the hypoglycemicc activity of G. subcaulescens.