A study on the phytochemical composition of the roots of Polygonum multiflorum collecting in Vietnam and an evaluation of its capacity to inhibit pancreatic lipase enzyme was conducted. In total, 12 metabolites (1–12) were isolated from anti-lipase activity-guided fractionation of P. multiflorum. The isolated compounds (1–12) were identified as trans-resveratrol (1), methyl protocatechuate (2), methyl gallate (3), catechin (4), epicatechin (5), myrciaphenone A (6), quercetin (7), kaempferol (8), apigenin (9), luteolin (10), tricin (11), and afzelin (12). This is the first time Compound 12 has been reported from P. multiflorum. However, 12 second metabolites (1–12) were tested in vitro to assess the anti-lipase potential. The results revealed Compound 7 exhibited the most significant lipase inhibition, with an IC50 value of 0.30 ± 0.04 μM. Following closely, Compounds 6, 8, 10, and 12 displayed IC50 values of 5.38 ± 0.65, 0.72 ± 0.10, 0.79 ± 0.19, and 1.22 ± 0.18 μM, respectively. For comparison, orlistat (positive control), the most common anti-obesity drug available on the market, has an IC50 value of 0.91 ± 0.10 μM. Molecular docking was applied to explore the affinity and interactions between active compounds 6–8, 10, and 12 and proteins associated with obesity, namely pancreatic lipase–colipase complex (PLCC) and pancreatic lipase-related protein 1 (PLRP1). In addition, absorption, distribution, metabolism, excretion, and toxicity (ADMET) were used in predictions. The results indicate that the active compounds isolated from P. multiflorum could be further in-depth research on lipase inhibition.