Wild blackberry (Rubus ulmifolius Schott) is a culinary and medicinal plant traditionally used to treat various ailments, including hypertension. This study evaluated the vasorelaxant effects of five crude leaf extracts of R. ulmifolius (hexane, dichloromethane, ethyl acetate, methanol, and aqueous), as well as the hypotensive and antioxidant activities of its methanolic extract (MERu), and analyzed its phytochemical profile. Crude extracts, obtained using a Soxhlet apparatus, were tested in vitro on isolated rat aortic rings precontracted with phenylephrine. The hypotensive effect of MERu was examined in vivo in normotensive rats, and its antioxidant activity was assessed using the DPPH assay. Total phenolic and tannin contents were quantified by the Folin–Ciocalteu and hide powder methods, respectively, while UHPLC-MS was used to identify its phytochemicals. All crude extracts induced concentration-dependent vasorelaxation, with MERu showing the strongest effect (59.31% relaxation at 10⁻¹ g/L). Intravenous MERu induced significant blood pressure reductions in rats, starting at 1 mg/kg. At 20 mg/kg, systolic, diastolic, and mean arterial pressures dropped by 38.61%, 51.58%, and 45.19%, respectively. MERu also demonstrated potent antioxidant activity and was rich in polyphenols, particularly tannins. Sixteen compounds were identified, notably rubanthrone A, a galloyl-bis-HHDP glucose derivative, ellagic acid, and quercetin-3-O-β-D-glucuronide. These results suggest that R. ulmifolius may have therapeutic potential for hypertension and exhibits promising characteristics as a functional food.