Objective: Learning and memory performance have been reported to be impaired in patients with Major Depressive Disorder (MDD). Impairments are associated with diminished psychosocial functioning. Based on the processing-speed theory, we aimed to examine whether processing speed mediates the relationship between depression status and verbal, visuo-spatial and working memory impairment. Methods: A neuropsychological test-battery was administered to 106 patients with current MDD, 119 patients with remitted MDD and 120 healthy controls to assess processing speed, learning and memory performance. To examine the impact of diagnosis status and processing speed on learning and memory performance, simple mediation models were computed. Results: Currently depressed patients with MDD showed partially slowed processing speed, impaired short-term verbal and visuo-spatial memory performance compared to healthy controls. A basic deficit in processing speed mediated the relationship between depression status and verbal, visuo-spatial, and working memory impairment. However, there was no processing speed or memory impairment in patients with remitted MDD. Conclusion: Processing speed is an important factor regarding learning and memory impairment in patients with current MDD. Thereby, our results highlight novel targets for treatment of diminished learning and memory performance via enhancement of processing speed using pharmacological as well as therapeutic interventions.