Adolescence is an exceptional period of life, not least in terms of social and brain development. Friends become increasingly important, susceptibility to peer rejection increases, and brain regions involved in social cognition are predicted to go through major changes. Adolescents’ social lives today may, to different extents, take place on social-media platforms online. This systematic review investigates how social-media use (SMU) affects adolescents’ brains. Out of 626 studies from the initial search, seven met the inclusion criteria. Out of these, five studies used functional magnetic resonance imaging, one study used diffusion tensor imaging, and one study used diffusion-weighted imaging. Functional findings suggest the reward circuit of the brain, as well as brain regions implicated in social cognition, to be involved in SMU. Activity in the nucleus accumbens was elicited by both giving and getting likes on posted pictures, whereas more SMU related to increased activity in the medial prefrontal cortex during physical self-judgement. Structural findings indicate frequent SMU to be associated with more reward sensitivity in terms of increased white matter in reward-processing pathways. These studies provide an initial understanding of the neural mechanisms of adolescents’ SMU. Future research is needed to draw inferences about how SMU affects the brains of adolescents.