Wearable activity trackers (wearables) may offer an affordable and accessible way of increasing child and adolescent physical activity (PA). Wearables have been shown to successfully increase adult PA, however less is known about their impact on child and adolescent PA. The aim of this review was to examine the acceptability, feasibility and effectiveness of wearables, and their potential mechanisms of action, for increasing PA, in 5- to 19-year olds. A systematic search was conducted of 7 databases (from the start date of each database to December 2019). The following eligibility criteria were used: (a) published in English; (b) included participants aged 5- to 19-years; (c) examined the use of a wearable within an intervention, acceptability or feasibility study; and (d) measured PA and/or experiences of using a wearable. The presence or absence of behaviour change techniques (BCTs), were coded, using the behaviour change technique taxonomy v1. A narrative review (effectiveness findings), and a thematic synthesis (acceptability and feasibility findings) were conducted. The heterogeneity of study samples, designs, and PA outcomes did not permit a meta-analysis. Thirty-three studies were included. There was some evidence that wearables increase steps, and moderate-to-vigorous-intensity PA and reduce sedentary behaviour. Some evidence suggested that multi-component interventions may be more effective than using a wearable alone. On average, 7.8 BCTs (range:2-12) were present in studies exploring the effectiveness of wearables. From the thematic synthesis, four analytical themes (14 subthemes) were identified: (1) perceived facilitators and barriers of using a wearable may impact device use, (2) affective attitude: feelings towards using wearables, (3) wearables ease of use, understanding of PA outputs, and perceived impact on PA varies between devices and individuals, and (4) perceived mechanisms of action underlying wearables impact on PA. Children and adolescents reported wearables increased their PA, via wearable features that promote self-monitoring, goal setting, feedback, and competition. However, barriers of wearable use (e.g. technical difficulties, novelty effect) were present. Thus, there is some evidence that wearables are an acceptable, feasible and effective way of increasing PA, in 5- to 19-year olds. Future interventions may benefit from incorporating multiple intervention components and more BCTs, and aiming to overcome barriers of wearable use. This review is registered with PROSPERO (CRD42020164506) and has not been published in a scientific journal.