Smartphones are omnipresent and offer real-time information on the go. Predominantly, adolescent girls have been found to be engaged in levels of physical activity (PA) below the daily recommended guideline of at least 60 minutes of moderate-to-vigorous intensity, and especially during weekends. Lack of sufficient PA can lead to a risk of contracting non-communicable diseases. The purpose of this exploratory study was to investigate the efficacy of using mobile applications (app) (i.e. MapMyFitness [MMF]) in changing adolescent girls’ PA behaviour during weekends. It also examined the perceived benefits, challenges and recommendations for using mobile apps. Thirty-six participants aged 15 years ( Mage = 14.9; SD = 0.30) from a convenience sample volunteered and took part in the present study, which spanned four weekends. Quantitative results showed a significant difference in the mean step count between experimental and control groups during week two, probably due to the novelty effect when the participants were introduced to the new MMF mobile app. Overall, the use of the MMF app seems to be able to attenuate the decline of adolescent girls’ PA level during weekends. Qualitative results revealed benefits of using mobile apps to promote PA, such as a sense of autonomy in selecting PA, the ability to view friends’ postings of PA and self-monitoring of PA. The study revealed specific challenges to using such PA mobile apps - in particular, factors such as the cumbersome and confusing functions in the app that discouraged users from performing PA. Recommendations included allowing users to customise their accounts, simplifying the app’s functions, and including rewards and videos as motivators to enhance users’ PA experience. Although mobile apps may have the potential to encourage participation in PA, a careful selection of mobile app functions is required to engage adolescent girls to continue to use it for PA.