Clinician optimism is an important factor in achieving treatment outcomes in counselling contexts. 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 1 month. The TOS was found to achieve acceptable reliability (Cronbach's α =.68) and yielded consistent scores over a one month period (r =.68, p <.01). Factor analyses revealed a 3-factor solution reflecting (1) General Treatment Outcome Expectancy, (2) Personal Treatment Outcome Expectancy, and (3) Pessimism. These findings support the utility of the TOS for research purposes, but further revision is recommended to enhance the reliability of the scale.