Previous research examining the factors associated with problem gambling treatment outcomes has examined client factors and to date, treatment characteristics, therapist factors, and client-therapist interactions have essentially remained unexplored. This study aimed to investigate how client engagement variables (client-rated therapeutic alliance, therapist-rated therapeutic alliance, number of sessions attended, and client commitment) relate to treatment outcomes (gambling and general functioning) in a sample of 475 treatment-seeking problem gambling clients using a series of hierarchical regression analyses. Client-rated therapeutic alliance predicted both gambling and general functioning outcomes, but therapist-rated therapeutic alliance only predicted general functioning outcomes. There was no significant relationship between number of sessions and outcomes, but client commitment predicted gambling outcomes. Client satisfaction only mediated the relationships between therapeutic alliance and treatment outcomes. Taken together, the findings indicate that client engagement characteristics, in particular the therapeutic relationship, are active agents of change in psychological interventions for problem gambling.