Objective: To examine the longitudinal associations among fitness App use and exercise self efficacy with physical activity in college students, and to test mediating role of exercise self efficacy, thereby providing evidence for colleges to utilize digital tools to promote physical activity among college students. Methods: From March 3-7，2025, convenience sampling was used to recruit 1 330 college students from six colleges in eastern, central, and western China. Three waves of data collection were conducted on March 10-14 (T1), April 21-25 (T2), and June 3-6 (T3), 2025. Fitness App Use Scale, Exercise Self efficacy Scale, and International Physical Activity Questionnaire-Short Form were used for survey. The relationship between variables was explored using Pearson correlation analysis and Spearman rank correlation analysis. Three wave cross lagged panel models and longitudinal mediation models were constructed to examine the relationships among the variables. Results: Positive correlations were found among fitness App use, exercise self efficacy, and physical activity at the same time points as well as across adjacent time points ( r =0.09-0.60, all P <0.01). The total metabolic equivalent of task from IPAQ-SF was positively correlated with average daily steps and device recorded moderate to vigorous physical activity time ( r =0.40-0.46, all P <0.01). After controlling for relevant covariates, the cross mediation model analysis revealed that T1 fitness App use positively predicted T2 exercise self efficacy ( β =0.34), and T2 exercise self efficacy positively predicted T3 physical activity ( β =0.27) among college students (both P < 0.01). The standardized indirect effect of exercise self efficacy was 0.09 (95% CI =0.07-0.12), accounting for 45.69% of the total effect. Conclusion The longitudinal association between fitness App use and subsequent physical activity among college students is partially mediated by exercise self efficacy, suggesting that exercise self efficacy may be an important psychological pathway linking fitness App use to subsequent physical activity.