In this explorative study, forty-seven patients with relapsing-remitting multiple sclerosis were randomized to a custom 6-week cognitive rehabilitation intervention (n = 23) using the BrainHQ^TM web-based platform and to a control group condition (n = 24). Cognitive rehabilitation intervention consisted of two 40-minute sessions per week. All patients were tested with the Brief International Cognitive Assessment for Multiple Sclerosis battery, the Stroop Color-Word Test, and the trail making test, while the Beck Depression Inventory - Fast Screen questionnaire was used as a measure of mood and the cognitive reserve index as a measure of cognitive reserve. We used the reliable change index, to calculate clinically meaningful changes of performance, and to discriminate between responders and non-responders of this intervention. Statistically significant improvement of the group receiving treatment was observed mainly on measures of verbal and non-verbal episodic memory and, to a lesser extent, on reading speed, selective attention/response inhibition, and visual attention. Verbal memory and visual attention improvements remained significant after considering the corrected for multiple comparisons level of significance. According to reliable change index scores, 12/23 (52.2%) of patients in the intervention group presented meaningful improvement in at least one measure (Greek Verbal Learning Test: 26%, Brief Visuospatial Memory Test-Revised: 17.4%, Stroop-Words test: 13%). This explorative study provides evidence that, at least in the short term, cognitive rehabilitation may improve the cognitive performance of multiple sclerosis patients.