Mindfulness‐based cognitive therapy (MBCT) is an innovative evidence‐based intervention in mental and somatic health care. Gaining knowledge of therapeutic factors associated with treatment outcome can improve MBCT. This study focused on predictors of treatment outcome of MBCT for cancer patients and examined whether group cohesion, therapeutic alliance, and therapist competence predicted reduction of psychological distress after MBCT for cancer patients. Moreover, it was examined whether therapist competence facilitated therapeutic alliance or group cohesion. Multilevel analyses were conducted on a subsample of patients collected in a larger randomized controlled trial on individual internet‐based versus group‐based MBCT versus treatment as usual in distressed cancer patients. The current analyses included the 84 patients who completed group‐based MBCT out of 120 patients who were randomized to group‐based MBCT. Group cohesion and therapist competence did not predict reduction in psychological distress, whereas therapeutic alliance did. In addition, therapist competence did not predict therapeutic alliance but was associated with reduced group cohesion. Our findings revealed that therapeutic alliance significantly contributed to reduction of psychological distress in MBCT for cancer patients. Elaborating the clinical implications of the predictive significance of therapeutic alliance might be of added value to enhance the potential effect of MBCT.