GOAL The purpose of this study was: (i) to design and fabricate a biomechanically-assistive garment which was sufficiently lightweight and low-profile to be worn underneath, or as, clothing, and then (ii) to perform human subject testing to assess the ability of the garment to offload the low back muscles during leaning and lifting. METHODS We designed a prototype garment which acts in parallel with the low back extensor muscles to reduce forces borne by the lumbar musculature. We then tested 8 healthy subjects while they performed common leaning and lifting tasks with and without the garment. We recorded muscle activity, body kinematics, and assistive forces. RESULTS The biomechanically-assistive garment offloaded the low back muscles, reducing erector spinae muscle activity by an average of 23-43% during leaning tasks, and 14-16% during lifting tasks. CONCLUSION Experimental findings in this study support the feasibility of using biomechanically-assistive garments to reduce low back muscle loading, which may help reduce injury risks or fatigue due to high or repetitive forces. SIGNIFICANCE Biomechanically-assistive garments may have broad societal appeal as a lightweight, unobtrusive and cost-effective means to mitigate low back loading in daily life.