Rumex dentatus L. is a flowering plant with promising therapeutic effects. This study investigated the antioxidant efficacy of phenolic compounds isolated from R. dentatus L. in vitro and by conducting density function theory (DFT) studies to explore the mechanisms of action. The antioxidant, anti-inflammatory and antidiabetic effects of polyphenols-rich R. dentatus extract (RDE) were investigated in type 2 diabetic rats. Phytochemical investigation of the aerial parts of R. dentatus resulted in the isolation of one new and seven known compounds isolated for the first time from this species. All isolated phenolics showed in vitro radical scavenging activity. The antioxidant activity of the compounds could be oriented by the hydrogen atom transfer and sequential proton loss electron transfer mechanisms in gas and water phases, respectively. In diabetic rats, RDE attenuated hyperglycemia, insulin resistance and liver injury and improved carbohydrate metabolism. RDE suppressed oxidative stress and inflammation and upregulated PPARγ. In silico molecular docking analysis revealed the binding affinity of the isolated compounds toward PPARγ. In conclusion, the computational calculations were correlated with the in vitro antioxidant activity of R. dentatus derived phenolics. R. dentatus attenuated hyperglycemia, liver injury, inflammation and oxidative stress, improved carbohydrate metabolism and upregulated PPARγ in diabetic rats.