Humans walk with relatively short strides when compared to other facultatively bipedal primates walking at the same dimensionless speed. However, the kinematic basis of this difference is unclear. Differences in non-sagittal plane pelvic motion may contribute to it, but to an unknown extent. While humans use small transverse-plane pelvic rotations during walking, the pelvic rotations of facultative bipeds are two to three times larger. Pelvic rotations contribute to anterior translation of the swing-side lower limb, thereby increasing bipedal stride length (‘the pelvic step’). Here, we determine the contribution of the ‘pelvic step’ to stride length in human and chimpanzee bipedal walking. Kinematic data were recorded over a wide speed range for humans (0.3 ms-1–2.0 ms-1) in order to match a range of self-selected speeds of bipedal chimpanzees. At similar dimensionless speeds, humans have dimensionless strides that are 20–30% shorter than those of bipedal chimpanzees, and a pelvic step that is 5–6 times smaller than that of bipedal chimpanzees. Smaller pelvic rotation magnitude in the transverse plane in humans accounts for a third (25–37%) of the difference in dimensionless stride length between human and bipedal chimpanzee walking. Reduced pelvic rotations may engender a decrease in cost of locomotion via reductions in overall body angular momentum, which likely outweighs the increase in stride length that results from a non-human-like pelvic step. These results suggest a constraint to lengthening strides in humans and likely early hominins. Funded by NSF grants SBE 0935321, SMA 1719432, and the Leakey Foundation.