Rumination is a well-known risk factor for the onset and recurrence of depressive episodes. Depressed individuals with a tendency to ruminate have been found to exhibit deficits in updating the contents of working memory. Thus, working memory training targeting updating-specific cognitive control processes may bear the potential to reduce ruminative tendencies. This registered clinical trial (ClinicalTrials.gov/NCT03011216) examined the effect of training cognitive control on rumination in the daily lives of clinically depressed individuals. Sixty-five individuals with a current major depressive disorder were randomized to 10 sessions of either cognitive control training (N=31) or placebo training (N=34). Primary outcome measures were the frequency and negativity of ruminative thoughts in the daily lives of participants assessed by a 7-day experience sampling procedure prior to training, after training, and at 3-months follow-up. Secondary outcomes were depressive symptoms, depressive mood, and level of disability. Cognitive control training led to stronger improvements in the trained task than placebo training. There was no transfer of the training effect to a novel cognitive control task. Cognitive control training did not lead to a greater reduction in daily rumination frequency, negativity of ruminative thoughts, or the negative influence of rumination on subsequent affect than the placebo training. There was no training-specific effect on participants’ depressive symptomatology or level of disability. Results join in a picture of mixed findings regarding the effect of cognitive control training on rumination. Future research has to identify determinants of beneficial training effects.