BACKGROUND: Previous research has demonstrated the clinical effectiveness of long-term psychological treatment for people with some types of personality disorder. However, the high intensity and cost of these interventions limit their availability. Lower-intensity interventions are increasingly being offered to people with personality disorder, but their clinical and cost effectiveness have not been properly tested in experimental studies. We therefore set out to develop a low intensity intervention for people with personality disorder and to test the feasibility of conducting a randomized controlled trial to compare the clinical effectiveness of this intervention with that of treatment as usual (TAU). METHODS: A two-arm, parallel-group, single-blind, randomized controlled trial of Psychological Support for Personality (PSP) versus TAU for people aged over 18 years, who are using secondary care mental health services and have personality disorder. We will exclude people with co-existing organic or psychotic mental disorders (dementia, bipolar affective disorder, delusional disorder, schizophrenia, schizoaffective disorder, or schizotypal disorder), those with cognitive or language difficulties that would preclude them from providing informed consent or compromise participation in study procedures, and those who are already receiving psychological treatment for personality disorder. Participants will be randomized via a remote system in a ratio of PSP to TAU of 1:1. Randomization will be stratified according to the referring team and gender of the participant. A single follow-up assessment will be conducted by masked researchers 24 weeks after randomization to assess mental health (using the Warwick and Edinburgh Well-Being Schedule), social functioning (using the Work and Social Adjustment Scale), health-related quality of life (EQ-5D-5 L), incidence of suicidal behavior, satisfaction with care (Client Satisfaction Questionnaire), and resource use and costs using a modified version of the Adult Service Use Schedule. In addition to this, each participant will be asked to complete the patient version of the Clinical Global Impression Scale. Feasibility and acceptability will primarily be judged by study recruitment rate and engagement and retention in treatment. The analysis will focus principally on descriptive data on the rate of recruitment, characteristics of participants, attrition, adherence to therapy, and follow-up. We will explore the distribution of study outcomes to investigate assumptions of normality in order to plan the analysis and sample size of a future definitive trial. DISCUSSION: Most people with personality disorder do not currently receive evidence-based interventions. While a number of high intensity psychological treatments have been shown to be effective, there is an urgent need to develop effective low intensity approaches to help people unable to use existing treatments. PSP is a low intensity intervention for individuals, which was developed following extensive consultation with users and providers of services for people with personality disorder. This study aims to examine the feasibility of a randomized trial of PSP compared to TAU for people with personality disorder. TRIAL REGISTRATION: ISRCTN Registry, ISRCTN14994755 . Registered on 18 July 2017.