Abstract: Background: Anxiety is highly prevalent in people diagnosed with bipolar disorder (BD), and can persist between acute episodes of mania and depression. Recent studies indicate that people with BD are prone to experiencing frequent, intrusive and emotional mental images which further fuel their levels of anxiety and mood instability. These intrusive emotional mental images represent a specific target for treatment for this disorder with the potential to reduce anxiety and improve mood stability. A new brief structured psychological intervention for BD called Imagery Based Emotion Regulation (IBER) has been developed, which translates experimental work in the area of imagery and emotion into a skills training programme to improve the regulation of intrusive and distressing emotional mental images in BD. A feasibility trial is required in order to assess whether a full randomised controlled trial is indicated in order to evaluate this approach. Methods: The design is a two-arm feasibility randomised controlled trial (RCT), with 1:1 randomisation stratified by trial site and minimised on medication status and anxiety severity. Participants are 60 individuals diagnosed with bipolar disorder and experiencing at least a mild level of anxiety. Sites are defined by the geographical boundaries of two National Health Service (NHS) Trusts, with recruitment from NHS teams, GP surgeries and self-referral. The intervention is up to 12 sessions of Imagery Based Emotion Regulation within 16 weeks. The comparator is NHS standard care. The primary aim is to assess the feasibility of conducting a powered multi-site RCT to evaluate effectiveness. Measures of anxiety, depression, mania, mood stability and health care use will be conducted at baseline, end of treatment and at 16-week follow-up. Discussion: This is the first feasibility trial of an imagery-based intervention for the treatment of anxiety in bipolar disorder. If the trial proves feasible, a large multi-site trial will be required. Trial registration: ISRCTN16321795. Registered on October 16, 2018. 10.1186/ISRCTN16321795