PURPOSE: Smartphone applications provide an opportunity for implementing physical activity (PA) interventions remotely. However, little research has been published to date on their effects. The purpose of the study was to test the effectiveness of efitbuddy, a theory-based PA smartphone application, on young adults’ PA and motivational beliefs through a four-week intervention. METHODS: A quasi-experimental design with control group was used to examine the effects of efitbuddy on participants’ PA. 274 college students (167 females, mean age = 19.35±2.09 years) attended baseline and posttests and change scores were computed for each dependent variable (DV). After the baseline test, participants in the intervention group (n=187) downloaded Efitbuddy and used it daily for four weeks. Efitbuddy was a smartphone application developed to promote individual’s PA and included four behavior change techniques such as self-monitoring, setting goals, and provision of general health information. PA participation, self-efficacy, and exercise enjoyment were selected as the DVs of the study. A Pearson’s correlation analysis was employed on the raw data to examine if the data were suitable for multivariate analyses. The results displayed moderate linear relationships between the pretest and posttest scores on three DVs. Therefore a 2 (group) x 2 (time) x 2 (gender) Multivariate Analysis of Variance (MANOVA) was conducted to examine the differences in the dependent variables. Wilk’s lambda was used to decide the statistical significance of the multivariate model. RESULTS: There were no multivariate statistically significant interactions for Group x Time x Gender (Wilks’s ? = 0.996, F(6, 269) = 0.687, P = 0.560), for Time x Gender (Wilks’s ? = 0.003, F(6, 269) = 0.508, P = 0.677), and for Group x Time (Wilks’s ? = 1, F(6, 269) = 0.008, P = 0.999). However, the results from the MANOVA yielded a significant interaction for Gender x Group (Wilks’s ? = 0.985 F(6, 269) = 2.720, P = 0.044). CONCLUSIONS: The results of the study demonstrate that efitbuddy has limited influence on young adults’ PA through a four-week period of time usage. These results echo previous studies exploring the relationship between smartphone applications and PA behaviors and more research is warranted for longer intervention with more vigorous engagement of usage.