OBJECTIVE: "Decentering" is defined as the ability to observe one's thoughts and feelings as temporary, objective events in the mind (Safran & Segal, 1990), and is increasingly regarded as a candidate mechanism in mindfulness-based interventions. The present study sought to examine the role of decentering, and other related variables, in the efficacy of Mindfulness-based cognitive therapy (MBCT) as compared to two active comparison conditions. METHOD: Formerly depressed individuals (N = 227), randomly assigned to MBCT (n = 74), relaxation group therapy (RGT; n = 77) or Treatment as usual (TAU; n = 76), completed self-report measures of decentering and symptoms of depression at pre-, mid-, and posttreatment, and relapse was assessed at 3, 6, 9, and 12 months, posttreatment. RESULTS: With regard to the acute treatment phase, results indicated that, whereas levels of depression increased in both RGT and TAU, MBCT patients remained free from symptom gains. Moreover, gains in decentering from mid- to posttreatment predicted reductions in depression from pre- to posttreatment for MBCT and TAU, but not for RGT. Participants who experienced increases in decentering, measured from mid- to posttreatment, generally evidenced the lowest levels of relapse/recurrence (during the four follow-up assessments), largely irrespective of treatment group. However, results related to change in decentering should be considered exploratory due to small cell sizes among participants who did not experience gains in decentering. CONCLUSIONS: Taken together, these results suggest that decentering is a potent mechanism for reduction of relapse in major depression, albeit one that is nonspecific to MBCT. (PsycInfo Database Record (c) 2022 APA, all rights reserved).