This study aimed to explore the effects of a single bout of light-intensity walking on sleep in older women with mild sleep impairment. A total of 40 women aged 55 years or older with mild sleep impairment were randomized to either a treadmill walking session for 50 minutes or a quiet-rest control. All participants completed the study (mean age: 60.4 ± 4.7 years). Sleep quality was assessed by ActiGraph for 2 nights before (pretest) and 2 nights after exercise (posttest). A mixed-design analysis of variance was used with group as the between-subjects factor and time point as the within-subjects factor. No significant group difference in demographic variables, body mass index, physical and mental status, and eight sleep parameters were observed at baseline. Significant group-time interactions existed for sleep latency (P < .001) and sleep efficiency (P = .025). After the intervention, the walking group reduced sleep latency by 3.3 minutes (P = .001) and also had greater sleep efficiency (increase 3.8%, P = .008), but no significant change was found in the control group. No significant group-time interactions were present for the other six sleep parameters (activity counts, total sleep time, wake after sleep onset, number and length of awakenings, or time in bed). A single session of light-intensity walking led to a modest reduction in sleep latency and improvement of sleep efficiency in older women with mild sleep impairment.