Lithospermum erythrorhizon (gromwell) root contains shikonin derivatives with anti-inflammatory and antioxidant properties, and preclinical studies have suggested protective effects on muscle tissue. However, clinical evidence in humans remains limited. Given that age-related loss of muscle strength contributes significantly to functional decline in older adults, effective and safe nutritional interventions are needed.This study aimed to evaluate the effects of gromwell root extract supplementation on muscle strength and physical function in adults aged 50 years or older with low skeletal muscle mass.In this 12-week, randomized, double-blind, placebo-controlled trial, 87 participants were assigned to receive either gromwell root extract (1000 mg/day) or placebo, in combination with supervised resistance exercise. The primary outcomes were peak torque of knee extension and flexion at 60°/s measured using isokinetic dynamometry. Secondary outcomes included lower-limb muscle power, body composition assessed by bioelectrical impedance analysis, handgrip strength, the Short Physical Performance Battery (SPPB), and the EuroQol-5 Dimension (EQ-5D). Safety was evaluated through adverse event monitoring and routine laboratory tests.Compared with the placebo group, participants receiving gromwell root extract showed significant improvements in right knee extension peak torque (+4.32 vs −0.90 Nm; p = 0.040) and right knee flexion peak torque (+2.92 vs −0.70 Nm; p = 0.041). Muscle power increased significantly in left knee extension (+3.66 W; p = 0.039), right knee extension (+4.63 W; p = 0.046), and right knee flexion (+2.78 W; p = 0.046). No significant between-group differences were observed in muscle mass, handgrip strength, SPPB scores, or EQ-5D scores. The supplement was well tolerated, with no reported adverse events.These findings indicate that gromwell root extract may enhance muscle function (strength and power) rather than muscle quantity, suggesting a possible neuromuscular or performance-related benefit when combined with resistance training. The lack of changes in muscle mass and global functional indices (SPPB, EQ-5D) may reflect the relatively short intervention duration and/or limited sensitivity of these outcomes in this population.Gromwell root extract improved lower-limb muscle strength and power when combined with resistance exercise and was well tolerated, supporting its potential as a safe nutritional adjunct for older adults with low muscle mass.