Growing research indicates that police legitimacy is a strong predictor of whether people behave respecting or violating rules. Perceptions of legitimacy are an output of socializing processes through which individuals develop their values and orientations toward authorities and the legal system. Legal socialization studies show that encounters with legal authorities are critical “teachable moments” in this process. The present study verifies whether direct or vicarious negative contacts with police officers affect changes in the perception of the legitimacy of police authority by adolescents over time. The adolescents were classified according to whether or not they had witnessed or experienced any negative contact or experience with the police during the period before the interview, composing two group trajectories at the first wave, four at the second wave, and eight at the third wave. Then the trajectories were compared in terms of the extent to which they agree with statements about police legitimacy, allowing the quantification of changes of opinion after negative contacts with the police. Results show that three main factors diminish the perception of police legitimacy: having negative contact with the police; having more than one negative contact; and having a recent negative contact. These findings have important implications for police patrolling and approach strategies.