This study examined the prevalence, characteristics and functions of Non-suicidal Self-injury (NSSI) among Spanish adolescents. The sample consisted of 1,864 adolescents aged between 12 and 19 years (Mean Age = 15.32, SD = 1.97, 51.45% girls). The participants completed a modified version of the self-report scale Functional Assessment of Self-Mutilation (FASM; Lloyd, Kelley, & Hope, 1997) to assess rates and methods of NSSI used during the last 12 months. They also indicated the functions of NSSI. NSSI behaviors are common among Spanish adolescents. More than half of the sample showed such behavior in the past year, and 32.2% had carried out severe NSSI behaviors. The functions of NSSI were examined by using confirmatory factor analyses. Results supported a hierarchical model consisting of two second-order factors: automatic reinforcement, which explained both positive and negative automatic reinforcement, and social reinforcement, which explained both positive and negative social reinforcement. These dimensions are critical to understand the factors that maintain NSSI behavior and have implications for treatments.