Background: Evidence suggests that adolescent males' physical activity levels are declining more rapidly than females. Adolescent males' motivation to be physically active needs to be better understood to ensure they stay active into adulthood. Purpose: The aim of the systematic review was to synthesize qualitative research framed by Self Determination Theory to examine adolescent males' motivation to be active. Methods: Databases were searched using key terms to identify studies that met the inclusion criteria. Results: Adolescent males' experienced physical activity in line with intrinsic and extrinsic regulations and felt amotivated to be active if they found physical activity boring, felt vulnerable in physical activity spaces, or experienced peer teasing. Peer relatedness was key to their motivation to be active, associated with both amotivation via peer comparison, and positively with increased intrinsic motivation to be active with friends and those of a similar ability. Discussion: Increased autonomy in adolescence means leisure-time physical activity with friends, away from school and parents, is critical if adolescent males are to remain active into adulthood. Translation to Health Education Practice: Adolescent males' physical activity needs to be inclusive to a multifaceted definition of masculinity, ensuring all adolescent males' basic psychological needs are met.