The purpose of the study was to examine the effectiveness of "efitbuddy," a theory-based smartphone application, on young adults' BMI, physical activity, and motivation across a 4-week intervention. Participants were a convenience sample of 312 (mean age = 19.4 ± 2.3 years) college students. A quasi-experimental design with the control group was used to examine the effects of "efitbuddy" on BMI, physical activity, and motivation. Participants in the intervention group downloaded "efitbuddy" and used it daily for a four-week period. Outcome measures included BMI, Physical Activity Questionnaire (PAQ) scores, self-efficacy, and enjoyment, collected at baseline and posttest timepoints. Mixed effects models yielded no significant improvements in BMI (p = 0.725), PAQ scores (p = 0.745), self-efficacy (p = 0.301), or enjoyment (p = 0.104). The 4-week "efitbuddy" intervention did not yield improvements in BMI, physical activity, or motivation in a sample of college students. These null findings are congruent with other smartphone interventions attempting to promote behavior change.