The split-belt treadmill (SBT) has been used to examine the adaptation of spatial and temporal gait parameters. Historically, SBT studies have focused on anterior-posterior (AP) spatiotemporal gait parameters because SBT is primarily a perturbation in the anterior-posterior direction, but it is important to understand whether and how ML control adapts in this scenario. The medial-lateral (ML) control of balance must be actively controlled and adapted in different walking environments. Furthermore, we seek to determine whether ML balance adaptation differs in older age. We analyzed SBT-induced changes in gait parameters including variables which inform us about ML balance control in younger and older adults. Our primary finding is that younger adults showed sustained asymmetric changes in these ML balance parameters during the split condition of the SBT. Specifically, younger adults sustained a greater displacement between their fast stance foot and their upper body, relative to the slow stance foot, in the ML direction. This finding suggests that younger adults may be exploiting passive dynamics in the ML direction during the SBT, which may be more metabolically efficient. Additionally, the ML parameters did not show any aftereffects despite large adjustments during the split condition, which may indicate higher level neural control than AP parameters.