Introduction: Extremism and the justification of violent and illegal acts among young people is one of the major concerns of democratic societies. We have not found any instrument that measures violent extremism among young people in Spain. The purpose of this study is to validate and evaluate the psychometric properties of the Spanish version of the scales of extremism and use of violence and illegal acts (Ozer & Bertelsen, 2018). Method: A sample of 1,135 high school students was used to validate both scales. A Social Extremism Index and a Violence and Illegality Justification Index were then developed to identify patterns of risk of violent extremism. Results: The construct validity of the two scales was tested by confirmatory factor analysis (CFA): Extremism Scale (CFI: 0.934, TLI: 0.919, NFI: 0.918, RMSEA: 0.050) and Pro-violence and illegal acts in relation to extremism Scale (CFI: 0.990, TLI: 0.985, NFI: 0.990, RMSEA: 0.050). The pro-violence and illegal acts in relation to extremism scale has been modified. After checking the good fit of the revised scales (CFA, reliability analysis and factorial invariance analysis), the social extremism index and the justification of violent illegal acts were calculated from their direct and percentile scores. The usefulness of the scales was confirmed by the characterization of the sample analyzed, which revealed that 15.59% (177) of the participants presented high or very high levels of violent extremism. Discussion and conclusion: The Spanish adaptation of the scale proposed by Ozer and Bertelson (2018) is useful for identifying the risk profile of social extremism and the degree of justification of violent-illegal acts among young Spanish secondary school students.