Aims and objectives To explore the therapeutic relationship through the reflective practice of nurses in acute mental health units. Background In mental health units, the therapeutic relationship is especially relevant for increasing the effectiveness of nursing interventions. Reflective practice is considered an essential aspect for improving nursing care. Design Action and observation stages of a participatory action research project. Methods Data were collected through reflective diaries designed for the guided description and reflection of practice interactions related to the therapeutic relationship and content analysis was applied. A total of 152 nurses from 18 acute mental health units participated. The COREQ guidelines were used. Results The results were classified into three categories as follows: (i) Nursing attitude as a core of the therapeutic relationship. For the nurses, the attitudinal component was key in the therapeutic relationship. (ii) Nursing practices that are essential to the therapeutic relationship. Nurses identified practices such as creating a conducive environment, using an appropriate verbal approach, offering help and working together with the patient as essential for establishing a therapeutic relationship in practice. (iii) Contextual factors affecting the therapeutic relationship. The nurses considered the patient's condition, the care dynamics of the unit and its regulations, as well as the structure and environment of the unit, as contextual factors involved the establishment of an adequate therapeutic relationship in daily clinical practice. Conclusions This study has provided knowledge of the importance and role of the nurses' attitude in the context of the nurse–patient therapeutic relationship based on the reflections of nurses in mental health units regarding their own practice. Relevance to clinical practice These findings help nurses to increase awareness and develop improvement strategies based on their own knowledge and day‐to‐day difficulties. Moreover, managers can evaluate strategies that promote motivation and facilitate the involvement of nurses to improve the therapeutic relationship with patients.