Adolescents are not sufficiently physically active. Previous attempt to promote physical activity via media interventions have been shown to have little effect. Potentially, interventions can be more effective when utilizing the impact that adolescents have on each other’s physical activity. Social network interventions are an emerging and promising approach to counteract the decline in physical activity, by capitalizing on the influence peers have on youth’s behaviors. In social network interventions, a small group of influential individuals (influence agents) is selected to promote health behaviors within their social network. However, no previous social network intervention study has investigated whether a social network intervention is more effective than a mass media campaign or no intervention. Adolescents (N = 446; Mage = 11.35 (SD = 1.34); 47.31% male) were randomly allocated by classroom (N = 26) to one of three conditions: social network intervention, mass media intervention or control condition. In the social network intervention, 15% of the participants was selected to be an influence based on peer nominations (on closeness centrality) and were asked to create several vlogs about physical activity. During the intervention period, participants in the social network condition were able to view the vlogs on a research smartphone. In the mass media intervention, participants were exposed to vlogs made by unfamiliar peers (i.e., the vlogs of the social network intervention). All participants received a research smartphone to complete questionnaires and a wrist-worn accelerometer to measure physical activity. Mixed effect models were us to account for the clustering of data per participant, per day, and per classroom. The analyses showed that there was an overal increase in physical activity druing the intervention. However, no differences were observed between the social network intervention and the control condition in the short-term, and an unexpected increase in the control condition compared to the social network intervention in the long-term. Also no differences between the social network intervention and mass media intervention were observed. In addition, exploratory analyses suggest that descriptive norms involving physical activity increased in the social network intervention and decreased in the mass media intervention. Also, participants in the social network intervention watched the vlogs more often and for a longer period, than participants in the mass media intervention that. The current study was the first study to test the additional benefit of implementing a social network approach the promote physical activity in adolescents by comparting a social network intervention to a mass media intervention and no intervention. The results do not provide evidence that the social network intervention was more effective in increasing physical activity in adolescents. Compared to other social network interventions, the current study did not train the influence agents how they could promote physical activity in the class, but merely showed them how they could create vlogs. Perhaps, a key factor to the effectiveness of social network interventions is a training of the influence agents.