Background Real-world implementation of psychological interventions for psychosis is poor.Barriers include therapy being insufficiently usable and useful for a diverse range of people.User-centred, inclusive design approaches could improve the usability of therapy, which may increase uptake, adherence and effectiveness.Objective To optimise the usability of an existing psychological intervention, Thinking Well, which targets reasoning processes in paranoia using a basic digital interface.Methods We conducted inclusive, user-centred design research characterised by purposive sampling of 'extreme' users from the margins of groups, ethnographic investigation of the problem context, and iterative prototyping of solutions. The UK Design Council's Double Diamond method was used. This consisted of 4 phases: discover, including a case series of Thinking Well, stakeholder interviews, desk research, user profiling, system mapping and a mood board; define, consisting of workshops to synthesise findings and generate the design brief; develop, involving concept workshops and prototype testing; and deliver, in which the final minimal viable product was storyboarded and iteratively coded.Results Consistent with our previous work, the Thinking Well case series showed medium/large effects on paranoia and wellbeing, and small effects on reasoning. These were maintained at follow-up despite some participants reporting difficulties with the therapy interface. Insights from the discover phase confirmed that usability was challenged by information complexity and poor accessibility. Participants were generally positive about the potential of technology to be enjoyable, help manage paranoia and provide tailored interpersonal support from therapists and peers, although reported privacy and security concerns. The define phase highlighted that the therapy redesign should support monitoring, simplify information processing, enhance enjoyment and trust, promote personalisation and normalisation, and offer flexible interpersonal support. During the develop phase over 60 concepts were created, with two key concepts of thoughts visualised as bubbles, and therapy as a journey selected for storyboarding. The output of the deliver phase was a minimal viable product of an innovative digital therapy, SlowMo. SlowMo works by helping people to notice their worries and fast thinking habits, and encourages them to slow down for a moment to find ways of feeling safer. A webapp supports the delivery of eight face-to-face sessions, which are synchronised to a native mobile app.Conclusions SlowMo makes use of personalisation, ambient information and visual metaphors to tailor the appeal, engagement and memorability of therapy to a diversity of needs. Feasibility testing has been promising, and the efficacy of SlowMo therapy is now being tested in a multi-centred randomised controlled trial. The study demonstrates that developments in psychological theory and techniques can be enhanced by improving the usability of the therapy interface in order to ...