Clinician optimism is an important factor in achieving treatment outcomes in psychotherapy. Currently there are no measures of mental health clinician optimism which report substantial psychometric validation. This study sought to assesses the validity and reliability of the Therapeutic Optimism Scale (TOS). 223 mental health clinicians working in a range of clinical settings were administered the TOS and convergent and discriminate validity were established. Test-retest reliability was established over a period of one month. The Therapeutic Optimism Scale was found to achieve acceptable reliability (Chronbach's alpha = .68) and yielded consistent scores over a one month period (r = .68, p < .01). Factor analyses revealed a three-factor solution reflecting (1) General Treatment Outcome Expectancy, (2) Personal Treatment Outcome Expectancy and (3) Pessimism. These findings support the utility of the Therapeutic Optimism Scale for research purposes; however further revision is recommended to enhance the reliability of the scale.