Neurocognitive impairments are observed in depression and associated with poor functioning. This study examined the efficacy and the effectiveness of cognitive remediation with supplemental Internet-based homework in treatment-resistant depression. Participants were randomized to treatment or wait list control conditions. Treatment consisted of 10 weeks of weekly group sessions and daily online cognitive exercises completed at home. The participants were assessed on cognitive, mood, motivation, and functioning measures. There was a significant time by treatment interaction for attention/processing speed and verbal memory. Changes in functioning were not significant, although improved cognition predicted improvements in functioning. Number of minutes of online exercise was associated with greater cognitive improvements. Cognitive deficits are malleable with behavioral treatment in a mood disorder characterized by severe and persistent symptoms.