Objectives This study sought to investigate the psychometric properties of the Posttraumatic Cognitions Inventory (PTCI; Foa et al., 1999, Psychol. Assess., 11, 303) among a cohort of older adolescents and to determine the relationship between post-traumatic cognitions and a variety of psychological outcomes including depression, anxiety, stress, and loneliness. Methods The PTCI was investigated among a large sample (N = 785) of Northern Irish adolescents. Confirmatory factor analysis and composite reliability analysis were conducted to assess the psychometric properties of the scale. Results The familiar three-factor solution of negative cognitions of self, negative cognitions of the world and others, and self-blame was supported; however, it was necessary to remove eight items from the original 33-item scale. The three-factor structure was subsequently demonstrated to be factorially invariant across gender and to possess satisfactory internal reliability. The three PTCI factors were found to correlate with depression, anxiety, stress, and three dimensions of loneliness. Conclusion These results provide the first piece of evidence that older adolescents cognitively respond to trauma in a similar manner to adults, that the PTCI is factorially invariant between genders, and that trauma cognitions are correlated with feelings of loneliness. The contextual dependent nature of the structure of the PTCI factors is discussed in relation to future research efforts. Practitioner points The PTCI is a valid and reliable measure of trauma-related cognitions among adolescents and works equally well for male adolescents and female adolescents. Trauma cognitions are associated with a range of mental health problems beyond post-traumatic stress disorder including depression, anxiety, stress, and various aspects of loneliness. Reductions in trauma cognitions in survivors of trauma will have wide-scale clinical benefits to patient well-being. The exact structure and make-up of items in the PTCI may well be dependent on culture, context, and the nature of the trauma. The study is limited due to the fact that the authors could not assess the severity of the trauma experienced by the adolescent sample.