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Introduction: Evidence-based psychological treatments for people with personality disorder usually involve attending group-based sessions over many months. Low-intensity psychological interventions of less than 6 months duration have been developed, but their clinical effectiveness and cost-effectiveness are unclear., Methods and Analysis: This is a multicentre, randomised, parallel-group, researcher-masked, superiority trial. Study participants will be aged 18 and over, have probable personality disorder and be treated by mental health staff in seven centres in England. We will exclude people who are: unwilling or unable to provide written informed consent, have a coexisting organic or psychotic mental disorder, or are already receiving psychological treatment for personality disorder or on a waiting list for such treatment. In the intervention group, participants will be offered up to 10 individual sessions of Structured Psychological Support. In the control group, participants will be offered treatment as usual plus a single session of personalised crisis planning. The primary outcome is social functioning measured over 12 months using total score on the Work and Social Adjustment Scale (WSAS). Secondary outcomes include mental health, suicidal behaviour, health-related quality of life, patient-rated global improvement and satisfaction, and resource use and costs. The primary analysis will compare WSAS scores across the 12-month period using a general linear mixed model adjusting for baseline scores, allocation group and study centre on an intention-to-treat basis. In a parallel process evaluation, we will analyse qualitative data from interviews with study participants, clinical staff and researchers to examine mechanisms of impact and contextual factors., Ethics and Dissemination: The study complies with the Helsinki Declaration II and is approved by the London-Bromley Research Ethics Committee (IRAS ID 315951). Study findings will be published in an open access peer-reviewed journal; and disseminated at national and international conferences., Trial Registration Number: ISRCTN13918289., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)