[Purpose] This study evaluated the effect of smartphone use on lumbar spine repositioning error and lumbar curvature while walking on a treadmill. [Subjects] A total of 20 healthy individuals (18 males and 2 females) volunteered for this study. [Methods] The subjects walked for 20 min on a treadmill while using a smartphone. To determine the effect of smartphone use, lumbar repositioning error was measured using an electronic goniometer while lumbar curvature was assessed using a Spinal Mouse before and immediately after treadmill use. Differences in the lumbar repositioning error and lumbar curvature data between the pre- and post-walking were compared using the paired t-test. [Results] The lumbar spine repositioning error was significantly greater post-walking compared with pre-walking (6.70±2.91° vs. 3.02±1.79°). There was no significant difference in lumbar curvature between pre- and post-walking (14.24±3.18° vs. 13.94±3.12°). [Conclusion] These findings indicate that the lumbar repositioning error increased immediately after walking while using a smartphone, but that the lumbar curvature was unchanged.