BACKGROUND: Therapeutic relationships are key to both service user recovery and the safety of staff and service users in adult mental health care. However, staff over-involvement (crossing professional boundaries including sexual and emotional exploitation) and under-involvement (staff disinterest, avoidance or neglect) is often a cause for concern within mental health care. Little is known about measuring and assessing over / under involvement. This scoping review provides a broad understanding of existing tools used to measure this in adult mental health care. OBJECTIVE: To explore what measures are used, and the characteristics of the identified measures, to understand the therapeutic relationship between staff and adult service users in mental health care settings. DESIGN: Scoping review. SETTING(S): Adult mental health settings. PARTICIPANTS: Service users and staff. METHODS: This review is guided by Levac et al.'s six stage methodology of scoping review frameworks. The reporting of this review has been guided by the PRISMA-ScR. RESULTS: Of 2863 papers found, 23 were eligible for inclusion. The papers identified 14 scales. No tool specifically measured over- or under- involvement. Finally, data indicates that scales should be specific to their intended setting as the nature of therapeutic relationships may vary by setting. CONCLUSIONS: Definitions of therapeutic relationships and over- and under-involvement relevant to different settings are needed. There is a need to develop setting-specific scales to measure therapeutic involvement and definitions for over- and under- involvement. This would enhance care provided to service users and encourage staff members to challenge their own boundary setting practices. REGISTRATION: https://osf.io/93dxp/.