Problem: Poor knowledge and stigmatizing attitudes of healthcare professionals constitute a significant barrier to child and adolescent mental health care worldwide. This study aimed to determine the effect of a training intervention on the knowledge and attitudes of pediatric nurses to child mental health problems at a Nigerian tertiary hospital. Methods: A two group pretest–posttest study design was undertaken. A total of 156 pediatric nurses were recruited, and participants in the intervention group received a brief child mental health training based on the World Health Organization's mhGAP training manual. Knowledge and attitudes to child mental health problems were obtained at baseline, and post intervention. Findings: There were no differences in knowledge or attitudes across the two groups at baseline. Post intervention, there was a significant increase in the mean post knowledge scores of the intervention group compared with the control group (t = 3.8, p <.001). The effect size of the intervention was 0.62. There were no significant differences in mean post attitude scores across groups. Conclusion: Incorporating children and adolescents mental health competencies into the training of nurses had a modest effect on their knowledge, but limited effect on their attitudes. [ABSTRACT FROM AUTHOR]