The beneficial impact of physical activity (PA) has been extensively documented, with well-evidenced improved physical and mental health across the lifespan, together with an increased life expectancy. Yet, a lack of PA participation and increased sedentary time continue to represent a serious public health burden. Insufficient levels of physical inactivity have also been observed in adolescents, which is alarming, as PA levels may be transferred into adulthood. Given the ubiquitous use of smartphones by adolescents, these devices may offer feasible means to reach young populations and deliver an intervention aimed at increasing PA participation. The aim of the current project, therefore, concerns the development, delivery and pilot-evaluation of a smartphone application intervention for adolescents who are insufficiently active, in order to promote PA engagement and decrease sedentary time.Existing research suggests that interventions, which have used behaviour change techniques (BCTs) were mostly theory-inspired rather than theory-based. Current mHealth applications do not often offer highly personalized features, although they could be important in increasing motivation and engagement. Additionally, research to date tends to focus on the development of applications for adults rather than adolescents, resulting in a general lack of research involving younger populations. The current project aims to address the above-mentioned gaps. The intervention (smartphone application and wrist-worn activity tracker) will be pilot-evaluated with a micro-randomized trial design. 36 adolescents aged 16-18 will participate in an 8-week trial. The primary outcome measure will be the number of daily steps taken and estimated daily minutes spent in moderate or vigorous PA. An additional evaluation of the intervention aiming at improving the application, based on user feedback, will be conducted after the trial. Differences in effectiveness of intervention components concerning participant age and gender will also be explored.