In the present study, we examined whether clients’ perceptions of similarity to their therapists in terms of the Big Five personality traits were associated with the real relationship and, indirectly through the real relationship, to treatment progress. Data collected through an online crowdsource platform from 212 adult clients in individual psychotherapy were analyzed using polynomial regression and response surface analysis. Results indicated that clients who perceived greater similarity to their therapists at higher (vs. lower) levels of Conscientiousness and Openness to Experience reported stronger real relationships, as did clients who perceived greater similarity at lower (vs. higher) levels of Neuroticism. Similarly, clients reported a stronger real relationship when they perceived their therapists as similarly high in Extraversion, but the real relationship decreased slightly when the similarity was perceived at higher (vs. moderate) levels of this trait. Perceived similarity at high and low levels of Agreeableness was also associated with stronger real relationships. There was no evidence of an indirect association between client-perceived similarity and treatment progress via the real relationship. However, clients who perceived greater similarity to their therapists at higher (vs. lower) levels of Openness to Experiences and Conscientiousness reported more progress in treatment. Implications for theory, research, and practice are discussed. (PsycInfo Database Record (c) 2021 APA, all rights reserved)