Objective To compare the efficacy of mindfulness‐based cognitive therapy (MBCT) plus treatment as usual (TAU) to TAU alone for patients with bipolar disorder over a 12‐month follow‐up period. Method Participants with a DSM‐IV diagnosis of bipolar disorder were randomly allocated to either MBCT plus TAU or TAU alone. Primary outcome measures were time to recurrence of a DSM‐IV major depressive, hypomanic or manic episode; the Montgomery‐Åsberg Depression Rating Scale (MADRS); and Young Mania Rating Scale (YMRS). Secondary outcome measures were number of recurrences, the Depression Anxiety Stress Scales (DASS), and the State Trait Anxiety Inventory (STAI). Results Ninety‐five participants with bipolar disorder were recruited to the study (MBCT = 48; TAU = 47). Intention‐to‐treat (ITT) analysis found no significant differences between the groups on either time to first recurrence of a mood episode or total number of recurrences over the 12‐month period. Furthermore, there were no significant between‐group differences on the MADRS or YMRS scales. A significant between‐group difference was found in STAI – state anxiety scores. There was a significant treatment by time interaction for the DAS – achievement subscale. Conclusion While MBCT did not lead to significant reductions in time to depressive or hypo/manic relapse, total number of episodes, or mood symptom severity at 12‐month follow‐up, there was some evidence for an effect on anxiety symptoms. This finding suggests a potential role of MBCT in reducing anxiety comorbid with bipolar disorder.