Background This review aimed to examine the relationship between motor/cognitive performance and cortical activity in individuals with multiple sclerosis (MS) under single- and dual-task conditions using functional near-infrared spectroscopy (fNIRS). Methods The review was preregistered with PROSPERO (CRD420261402862) and followed the PRISMA protocol. PubMed, Cochrane Library, and Web of Science were systematically searched, and 18 relevant studies were included to address four core questions: (1) whether cortical activation differs between MS patients and healthy controls; (2) how activation patterns vary under single- vs. dual-task conditions; (3) whether activation is associated with motor/cognitive performance; and (4) the potential mechanisms underlying altered cortical activation. Results The findings suggest that some MS patients may exhibit increased cortical activation alongside reduced performance. During single tasks, activation in motor/cognitive regions was often higher than that observed in healthy controls, and in some studies, this activation correlated with the severity of disability. This pattern became more pronounced under dual-task conditions, with concurrent declines in gait and cognitive performance reported. Rehabilitative interventions may help modulate such activation. Conclusions Certain MS patients demonstrate increased task-related cortical activation accompanied by reduced performance. While this may reflect alterations in neural efficiency or neurovascular coupling, it could also indicate compensatory recruitment or increased attentional effort, depending on the task, disability severity, and disease-related factors. fNIRS may serve as a useful monitoring tool, and future multimodal EEG-fNIRS research is warranted to clarify the underlying mechanisms. Systematic Review Registration https://www.crd.york.ac.uk/prospero/ , identifier CRD420261402862.