Lumbar lordosis (LL) is recognised as a factor affecting the mechanical load on the lumbar spine. As the forces and torques on the lumbar spine increase with running speed, assessment of LL may become increasingly important for injury prevention. Nevertheless, published data on LL during running are limited to a single slow speed. Here we show the response of LL to different running speeds, from slow running to sprinting. We recorded kinematic data of treadmill running at 4.0, 5.5, 7.0, and 8.5 m/s from 18 male athletes using a motion capture system, and evaluated LL based on markers placed on the T12–L5 spinous processes. LL significantly increased at each increment in running speed (peak Cobb-angle-like metric:​ from 26.4 ± 8.6 to 32.5 ± 9.5 deg), potentially increasing the mechanical load on the lumbar spine. Meanwhile, anterior pelvic tilt showed no significant difference across all running speeds. Lumbo-pelvic extension in a single-segment lumbar spine model decreased as running speed increased. Although anterior pelvic tilt and lumbo-pelvic extension are recognised as indicators of LL, their responses to changes in running speed differed from that of LL, indicating that greater lordosis is not always accompanied by increases in anterior pelvic tilt or lumbo-pelvic extension. Given that increased LL during faster running may contribute to greater mechanical load on the lumbar spine, we highlight the importance of direct assessment of LL during running, especially at higher speeds.