Objectives: Recently, use of the Hybrid Assistive Limb (HAL) that is effective for improvement of gait ability in chronic stroke patients has been reported. However, how long the effects are maintained remains unknown. The purpose of the present study was to investigate whether the effect of gait training using the HAL on gait ability was maintained for 3 months after the intervention. Methods: A longitudinal, observational study with an intervention for a single group that adhered to the STROBE guidelines was performed. Nine chronic stroke patients were enrolled in this study. The patients performed gait training sessions using the HAL, 2-5 sessions/week for 3 weeks. Gait speed, stride length, cadence, and 2-minute walk distance (2MWD) were measured before and after intervention and at 3-month follow-up. The clinical trial registration number of this study is UMIN000012764 R000014756. Results: Compared to the initial status, gait speed (p = .02), stride length (p = .03), cadence (p = .01), and 2MWD (p < .05) were significantly increased immediately after the intervention. Moreover, gait speed (p < .01), cadence (p = .03), and 2MWD (p = .02) remained significantly higher 3 months after the intervention. There were no significant changes in all outcome measures between after intervention and at 3-month follow-up. Conclusions: This study showed that gait training using the HAL resulted in significant improvement of gait ability after the intervention and the effect was maintained for 3 months after the training.