Individuals with borderline personality disorder (BPD) appear more likely than individuals with other mental disorders to evoke negative countertransference reactions. The current study examined countertransference toward BPD across client- (e.g., client age and gender) and clinician-level (e.g., age, discipline, clinical experience, training) factors. Participants (N = 560) completed an anonymous online survey in which they read case information describing a client with BPD and answered questions to assess their reactions toward the client. The study used a 2 × 2 between-subjects design in which client age and gender were experimentally manipulated. Despite receiving the same vignette, clinicians were more accurate in diagnosing the female client with BPD than the male client, and clinician reactions differed as a function of client age and clinician experience. Specifically, clinicians viewed adolescent clients with BPD as less ill, less trustworthy, and more dangerous than adults with BPD; more clinical experience among clinicians was associated with more positive reactions to clients. Findings help to better understand countertransference reactions and the ways they may impact diagnostic choices and treatment decisions. The implications of these findings for facilitating better clinician–client matching, reducing clinician burnout, and improving treatment experiences for individuals with BPD are discussed.