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Abstract Background Multiple sclerosis (MS) is a disabling disease of the central nervous system. People living with MS often have co-existing sleep disorders and cognitive dysfunction. The objective of this study was to scrutinize the relationship between cognitive outcomes and sleep conditions in MS. Methods This study followed the Joanna Briggs Institute’s (JBI) and PRISMA guidelines. PubMed, Scopus, Embase, and Web of Science databases were searched and original studies delineating the relationship between sleep status and cognitive findings in MS patients‌ were included. The risk of bias was assessed using the JBI critical appraisal tools. Results In the final review, out of 1635 screened records, 35 studies with 5321 participants were included. Pittsburgh Sleep Quality Index (PSQI), Epworth Sleepiness Scale (ESS), and polysomnography were the most common assessment tools for evaluation of sleep condition, and cognitive evaluations were conducted using the tests including Paced Auditory Serial Addition Test (PASAT), California Verbal Learning Test (CVLT), Symbol Digit Modalities Test (SDMT) and Brief Visuospatial Memory Test (BVMT). Assessing the quality of studies showed no significant bias in most of the included articles. A link between sleep condition and cognitive abilities was suggested in the literature, especially with objective measurement of sleep condition; however, current evidence did not support a substantial association between self-reported sleep quality and processing speed and working memory in patients with MS. Discussion Evidence proposes sleep is an independent factor associated with cognitive outcomes in MS. Given the limitations of the evidence such as the lack of well-designed prospective studies, these findings need to be interpreted with caution.