The present study assessed motivation for engaging in treatment as rated by clinicians (n = 57) and patients with severe mental illness (SMI, n = 294) using measures based on three different motivation theories. Questionnaires were derived from self‐determination theory, the transtheoretical model and the integral model of treatment motivation. It was investigated to which extent clinicians of patients with SMI were able to estimate their patient's perspective on motivation for engaging in treatment, to which extent they agreed on the patient's motivation and which factors were associated with estimation and agreement on treatment motivation. It was found that clinicians were poorly to moderately capable of estimating their patient's type of motivation and readiness for change. Further, agreement on the level of motivation between patients and clinicians was moderate. These findings were consistent across diagnostic groups (psychotic and personality disorders). A higher quality therapeutic relationship was generally associated with higher clinician‐rated motivation. The patient's ethnicity and socially desirable responding were factors that differentiated between scales of different motivation theories. It is concluded that patients with SMI and their clinicians have different perceptions on the patient's motivation for engaging in psychiatric treatment, regardless of the theoretical framework that is used to measure motivation. The findings imply that a negotiated approach is needed where both perceptions of clinicians and patients on motivation for treatment are considered to ensure effective mental health interventions. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message Clinicians show poor to moderate capability in estimating how patients perceive their motivation for engaging in treatment, especially so when the patient's motives revolve around feelings of shame and guilt. Clinicians generally give higher motivation ratings for patients where they experience a higher quality therapeutic relationships with, whereas—depending on the scale that is used to measure motivation—they give lower ratings to patients who respond in socially desirable ways and to ethnic minority patients. As patients with SMI and their clinicians have different perceptions on the patient's motivation for engaging in psychiatric treatment (regardless of the theoretical framework that is used to assess motivation), this implies that a negotiated approach is needed where both perceptions of clinicians and patients on motivation for treatment are considered to ensure effective mental health interventions.