This study investigates the relationships between therapists' use of discourse particles and therapist empathy. Discourse particles, commonly found in non-English languages, are verbal elements that constitute metacommunication by encoding speakers' emotions and attitudes, which are typically expressed by nonverbal behaviors (e.g., intonation, tone, facial expression, nodding). We hypothesize an inverted U-shaped curvilinear relationship between therapists' use of discourse particles and therapist empathy, given the notion that an optimal level of therapists' emotion in psychotherapy can facilitate clients' inner experiencing and self-expression. Four psychotherapy sessions each from 39 therapist-client dyads were analyzed. After each session, therapist empathy was rated by trained observers using the Therapist Empathy Scale (TES) and by clients using the Barrett-Lennard Relationship Inventory (BLRI). Multilevel modeling shows that both the person-level negative quadratic term and positive linear term for therapists' usage of discourse particles are significant in predicting mean TES with large effect sizes. The same predictors do not yield significant results in predicting mean BLRI but they trend in similar directions of associations with medium effect sizes. Our results suggest the optimal usage of discourse particles by therapists is around 20.3% (out of all utterances). The nonsignificant results in BLRI may be attributed to the relatively small sample size of our data and the noncommunication orientation of the client-rated measure. (PsycInfo Database Record (c) 2023 APA, all rights reserved).