Background: Psychiatric nurses are aware of the importance of the therapeutic relationship in psychiatric units. Nevertheless, a review of the scientific evidence indicates that theoretical knowledge alone is insufficient to establish an adequate therapeutic alliance. Therefore, strategies are required to promote changes to enhance the establishment of the working relationship. The aims of the study are to generate changes in how nurses establish the therapeutic relationship in acute psychiatric units, based on participative action research and to evaluate the effectiveness of the implementation of evidence through this method. Methods/Design: The study will use a mixed method design. Qualitative methodology, through participative action research, will be employed to implement scientific evidence on the therapeutic relationship. A quasi-experimental, one-group, pre-test/post-test design will also be used to quantitatively measure the effectiveness of the implementation of the evidence. Participants will consist of nurses and patients from two psychiatric units in Barcelona. Nurses will be selected by theoretical sampling, and patients assigned to each nurses will be selected by consecutive sampling. Qualitative data will be gathered through discussion groups and field diaries. Quantitative data will be collected through the Working Alliance Inventory and the Interpersonal Reactivity Index. Qualitative data will be analysed through the technique of content analysis and quantitative data through descriptive and inferential statistics. Discussion: This study will help to understand the process of change in a nursing team working in an inpatient psychiatric ward and will allow nurses to generate knowledge, identify difficulties, and establish strategies to implement change, as well as to assess whether the quality of the care they provide shows a qualitative improvement.