Abstract Background The COVID-19 pandemic changed lifestyles and raised concerns regarding physical inactivity among adults. Mobile health interventions for physical activity may be a potential solution. However, the effects may diminish over time. This study investigated whether incorporating non-health-targeted promotions into an existing mobile health-walking program could stimulate users to walk. Methods We used nationwide cohort data of smartphone users participating in a shopping mall walking program in Japan. We focused on existing users registered in the program until October 31, 2022. A Japanese anime tie-in promotion was conducted by the shopping mall company from December 3 to 25, 2022, offering additional rewards alongside the regular program. We collected step count records of existing users 1 month before and after (November 1 to December 2, 2022, and December 26, 2022 to January 31, 2023, respectively), and during the promotion period. Propensity score matching was applied to ensure balance in the baseline characteristics of existing users in terms of gender, age, and pre-promotion program participation frequency. Next, we analyzed the association between participation in the promotion program and the average daily step counts during and after the promotional period using mixed effects models for repeated measures, while adjusting for the same covariates. Results Among the 18,156 users included in the study, 13,386 (73.7%) were women, with a mean (SD) age of 45.2 (12.7) years, and a mean (SD) pre-promotion program participation frequency of 9.7 (9.6) per month. During the promotion period, the participants walked 87 (95% CI: 50-125) more steps per day than non-participants, after adjusting for covariates. No clear association was observed 1 month post-promotion. Conclusions These findings indicate that non-health-targeted promotions may be short-term motivators for existing app users to walk. Ongoing monitoring is necessary to assess the long-term impact. Key messages • Mobile health intervention effects may diminish over time. • A non-health-related promotion could stimulate existing mall walking program app users to increase their daily step count temporarily.