This study will test whether it is feasible to conduct a clinical trial of mirtazapine (an antidepressant tablet) in patients who have both depression and inflammatory bowel disease (IBD). The study design is a randomised controlled trial (a study in which people are allocated by chance to receive different interventions). The trial will compare mirtazapine against a placebo (dummy) tablet in 76 patients with both depression and IBD. The investigators will recruit outpatients aged 18 or over with a diagnosis of any IBD attending gastroenterology clinics. Either in person or remotely, patients will complete a brief screening questionnaire for depression. Those scoring positive for depression will be invited for a 15-minute interview for clinical depression. Those with clinical depression will be invited to take part. Participants will be randomly allocated by a computer to take either 1) mirtazapine tablet once at night for 12 weeks; or 2) placebo (dummy) tablet once at night for 12 weeks. The study is 'blinded', meaning neither patients nor the study team will know which medication they are taking. Throughout, participants will be able to access other treatments for depression, such as talking therapies. The investigators will measure how many people join the study; how many remain in the trial; how many complete treatment; how many tablets people take; and assess overall acceptability of the trial. Participants will complete brief questionnaires to measure their mental health and IBD symptoms after 4 weeks, 8 weeks, 12 weeks and 16 weeks. Participants will also provide blood samples and faecal samples to measure inflammation. If successful, this trial will support an application for a larger version of the study.