Bullying is a public health menace of global significance. Personality traits have been shown to predict bullying roles. This study aimed to assess the prevalence of bullying and its relationship with the Big-Five personality traits among Nigerian in-school adolescents. Four hundred and thirty-two adolescents aged between 12 and 18 years (M = 15.32, SD = ± 1.58) were randomly selected from six secondary schools in Ilesa, Nigeria. Personality traits were assessed with the Big-Five Personality Inventory while bullying was assessed with the peer relationship questionnaire. The prevalence of bullying behaviour among the respondents was high at 85.4%. There was a statistically significant difference (p = .001) between perpetrators and non-perpetrators for the neuroticism trait. On regression analysis, agreeableness trait was predictive of bullying behaviour as perpetrator (B = -0.090, SE = .023, [beta] = -.0231, t = -3.943 95% C.I. [-0.135, -0.045] p < 0.001) and as a victim (B = -0.138, SE = 0.026, [beta] = -0.310, t = -5.384 95% C.I. [-.188, -0.088] p < 0.001). There is a high prevalence of bullying among Nigerian in-school adolescents. Agreeableness trait predicted bullying behaviour either as perpetrators or victims. Our findings highlight the need for interventions targeting personality traits to prevent and fight school bullying.