Objective: Depression and suicidal ideation are closely intertwined. Yet, among young people with depression, the specific factors that contribute to changes in suicidal ideation over time are uncertain. Factors other than depressive symptom severity, such as comorbid psychopathology and personality traits, might be important contributors. Our aim was to identify contributors to fluctuations in suicidal ideation severity over a 12-week period in young people with major depressive disorder receiving cognitive behavioural therapy. Methods: Data were drawn from two 12-week randomised, placebo-controlled treatment trials. Participants (N = 283) were 15–25 years old, with moderate to severe major depressive disorder. The primary outcome measure was the Suicidal Ideation Questionnaire, administered at baseline and weeks 4, 8 and 12. A series of linear mixed models was conducted to examine the relationship between Suicidal Ideation Questionnaire score and demographic characteristics, comorbid psychopathology, personality traits and alcohol use. Results: Depression and anxiety symptom severity, and trait anxiety, independently predicted higher suicidal ideation, after adjusting for the effects of time, demographics, affective instability, non-suicidal self-injury and alcohol use. Conclusions: Both state and trait anxiety are important longitudinal correlates of suicidal ideation in depressed young people receiving cognitive behavioural therapy, independent of depression severity. Reducing acute psychological distress, through reducing depression and anxiety symptom severity, is important, but interventions aimed at treating trait anxiety could also potentially be an effective intervention approach for suicidal ideation in young people with depression.