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Background/Objectives: Diabetes mellitus is a chronic metabolic disorder characterized by hyperglycemia, dyslipidemia, and oxidative stress, leading to severe systemic complications. Medicinal plants rich in polyphenolic compounds have gained increasing attention as complementary therapeutic agents. This study aimed to comparatively evaluate the chemical composition, as well as the antidiabetic, hypolipidemic, and antioxidant effects of Polygonum persicaria and Vaccinium myrtillus in a streptozotocin-induced diabetic model. Although Vaccinium myrtillus has been more extensively investigated for its antidiabetic potential, the pharmacological relevance of Polygonum persicaria in diabetes remains insufficiently characterized, particularly in direct comparison with a recognized phytotherapeutic comparator. Methods: Hydroalcoholic tinctures prepared from Polygonum persicaria L. herb and Vaccinium myrtillus L. leaves were subjected to phytochemical analysis using High-Performance Thin-Layer Chromatography (HPTLC) for the identification of flavonoids and phenolcarboxylic acids, alongside spectrophotometric determination of total polyphenol and flavonoid content. Experimental diabetes was induced in CD1 mice by streptozotocin administration. Animals were treated orally for 35 days, and glycemic parameters, lipid profile, body weight, food and water intake, and oxidative stress markers (MDA, SOD, TAC, and GPx) were evaluated. Results: HPTLC/CSS screening indicated the presence of rutin, chlorogenic acid, and caffeic acid in Polygonum persicaria, while Vaccinium myrtillus showed stronger densitometric signals for phenolcarboxylic acid-type compounds, particularly chlorogenic and caffeic acids. Total polyphenol and flavonoid content were also higher in Vaccinium myrtillus (433.89 ± 8.67 mg/L GAE; 154.38 ± 3.08 mg/L QE) compared to Polygonum persicaria (269.28 ± 5.25 mg/L GAE; 132.75 ± 2.65 mg/L QE). Functionally, Vaccinium myrtillus demonstrated a significant antihyperglycemic effect from day 14 (p = 0.009) and improved lipid parameters, while Polygonum persicaria showed a delayed glycemic effect, significant only at day 35 (p = 0.014), without significant hypolipidemic activity. In contrast, Polygonum persicaria exerted a marked antioxidant effect, significantly increasing GPx activity (p = 0.025) and reducing MDA levels (p = 0.053). Conclusions:Vaccinium myrtillus showed stronger antihyperglycemic and hypolipidemic effects, while Polygonum persicaria was mainly associated with antioxidant-related biochemical changes. These differences may be influenced by phytochemical composition, but they cannot be attributed solely to total polyphenol or flavonoid content.