BACKGROUND: Current cognitive monitoring of people with multiple sclerosis (pwMS) is sporadic, resource intensive and insensitive for detection of real-world cognitive performance and decline. Smartphone applications may provide us with a more sensitive biomarker for cognitive decline that reflects real-world performance. The goal of this study was to perform a systematic review and qualitative synthesis of all current smartphone apps monitoring cognition in pwMS. METHODS: A systematic search of five major online databases (PubMed/Medline, Scopus, Web of Science, Cumulative Index of Nursing and Allied Health Literature and IEEE Xplore) was performed in accordance with the Cochrane Handbook and Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement. We included all studies with at least one measure of phone-based digital biomarkers for monitoring cognition in pwMS above the age of 18. Two authors independently screened the articles retrieved. Data on test-retest reliability, validity coefficients, feasibility and practice effects were extracted from the studies identified. Critical appraisal of the studies was performed using the National Institute of Health quality assessment tool for observational cohort and cross-sectional studies. RESULTS: 12 articles covering six smartphone apps were included in this review. All articles had a low risk of bias, though sample size calculation was rarely performed. Of the six apps, five used smartphone versions of the symbol digit modalities test. The final app examined keystroke features passively. Test-retest reliability ranged from good to excellent. Concurrent validity was demonstrated through moderate to strong correlation with neuropsychological tests and weak to moderate correlations with EDSS, radiological biomarkers and patient-reported outcomes. Mobile apps performed comparably, and in some cases outperformed established cognitive tests. Whilst reported acceptability was high, significant attrition rates were present in longitudinal cohorts. There were significant short and long-term practice effects. Overall, smartphone versions of the SDMT showed strong psychometric properties across multiple apps. CONCLUSION: Smartphone applications are reliable and valid biomarkers of real-world cognition in pwMS. Further longitudinal data would allow for a better understanding of their predictive and ecological validity.