Jerusalem artichoke is a low-requirement crop, which does not interfere with food chain, and is a promising carbon source for industrial fermentation. Microbial conversion of such a renewable raw material to useful products, such as lactic acid, is an important objective in industrial biotechnology. In this study, high-optical-purity l-lactate was efficiently produced from the hydrolysates of Jerusalem artichoke powder by a thermophilic bacterium, Bacillus coagulans XZL4. High l-lactate production (134 g l−1) was obtained using 267 g l−1 Jerusalem artichoke powder (total reducing sugars of 140 g l−1) and 10 g l−1 of corn steep powder in fed-batch fermentation, with an average productivity of 2.5 g l−1 h−1 and a yield of 0.96 g g−1 reducing sugars. The final product optical purity is 99%, which meets the requirement of lactic acid polymerization. Our study represents a cost-effective and promising method for polymer-grade l-lactate production using a cheap raw bio-resource.