ObjectiveTo evaluate the impact of mental health comorbidity in children and young adults with inflammatory bowel disease (IBD).DesignRetrospective observational study.SettingRepresentative population, routinely collected primary care data from the UK Optimum Patient Care Research Database (2015–2019).ParticipantsPatients with IBD aged 5–25 years with mental health conditions were compared with patients with IBD of the same age without mental health conditions.Primary and secondary outcome measuresOutcomes comprised quality-of-life indicators (low mood, self-harm, parasuicide, bowel symptoms, absence from school or work, unemployment, substance use and sleep disturbance), IBD interventions (medication, abdominal surgery, stoma formation and nutritional supplements) and healthcare utilisation (primary care interactions and hospital admissions).ResultsOf 1943 individuals aged 5–25 years with IBD, 295 (15%) had a mental health comorbidity. Mental health comorbidity was associated with increased bowel symptoms (adjusted incident rate ratio (aIRR) 1.82; 95% CI 1.33 to 2.52), sleep disturbance (adjusted HR (aHR) 1.63; 95% CI 1.02 to 2.62), substance use (aHR 3.63; 95% CI 1.69 to 7.78), primary care interactions (aIRR 1.33; 95% CI 1.12 to 1.58) and hospital admissions (aIRR 1.87; 95%CI 1.29 to 2.75). In individuals ≥18 years old, mental health comorbidity was associated with increased time off work (aHR 1.55; 95% CI 1.21 to 1.99).ConclusionsMental health comorbidity in children and young adults with IBD is associated with poorer quality of life, higher healthcare utilisation and more time off work. It is imperative that affected young patients with IBD are monitored and receive early mental health support as part of their multidisciplinary care.Trial registration numberThe study protocol was specified and registered a priori (ClinicalTrials.gov study identifier: NCT05206734).