Objectives. The aim of this study was to explore possible relationships between the HEXACO personality traits (i.e., Extraversion, Emotionality, Honesty-Humility, Agreeableness, Conscientiousness, Openness to experience) and negative effects of psychotherapy. Sample and settings. A total of 148 respondents answered an online survey. Descriptive and multivariate regression analyses were performed. Hypotheses. Clients’ personality traits were expected to predict negative effects of psychotherapy. Results. Extraversion was the strongest negative predictor of negative effects, serving as a protective factor against worsening of symptoms, hopelessness, and stigma. Furthermore, Emotionality was related to lower perceived quality of psychotherapy, Conscientiousness was related to hopelessness, and Openness to experience served as a protector against dependency. Limitations. The findings are based on a onetime retrospective measurement and a convenience sample. The results must be interpreted with caution because the assumptions of linear regression were not fulfilled. [ABSTRACT FROM AUTHOR]