Background: Smoking is highly prevalent among people who have experience of severe mental ill health, contributing to their poor physical health. Despite the 'culture' of smoking in mental health services, people with severe mental ill health often express a desire to quit smoking; however, the services currently available to aid quitting are those which are widely available to the general population and may not be suitable or effective for people with severe mental ill health. The aim of this study is to explore the effectiveness and cost-effectiveness of a bespoke smoking-cessation intervention specifically targeted at people with severe mental ill health. Methods/design: SCIMITAR+ is a multicentre, pragmatic, two-arm, parallel-group, individually randomised controlled trial. We aim to recruit 400 participants aged 18 years and above with a documented diagnosis of bipolar disorder, schizophrenia or schizoaffective disorder who smoke. Potentially eligible participants identified in primary or secondary care will be screened, and baseline data collected. Eligible, consenting participants will be randomly allocated to one of two groups. In the intervention arm, the participant will be assigned a mental health professional trained to deliver smoking-cessation interventions who will work with the participant and participant's GP or mental health specialist to provide an individually tailored smoking-cessation service. The comparator arm will be usual care - following current NICE guidelines for smoking cessation, in line with general guidance that is offered to all smokers, with no specific adaptation or enhancement in relation to severe mental ill health. The primary outcome will be self-reported smoking cessation at 12 months verified by expired carbon monoxide (CO) measurement. Secondary outcome measures include Body Mass Index at 12 months, the Fagerstrom Test for Nicotine Dependence, Motivation to Quit questionnaire, SF-12, PHQ-9, GAD-7, EQ-5D-5 L, and health service utilisation at 6 and 12 months. The economic evaluation at 12 months will be conducted in the form of an incremental cost-effectiveness analysis. Discussion: SCIMITAR+ trial is the largest trial to our knowledge to investigate the effectiveness of a bespoke smoking-cessation service for people with severe mental ill health.