Background and Objectives: Multiple sclerosis (MS) is a chronic autoimmune, inflammatory, and neurodegenerative central nervous system disease. Neurodegeneration plays a central role in long-term disease progression. Materials and Methods: This cross-sectional study examined the relationship between neurodegenerative biomarkers, namely plasma neurofilament light chain (pNfL) levels and MRI-derived brain volume measurements, and clinical outcomes in 49 patients with relapsing–remitting multiple sclerosis (RRMS). Plasma NfL levels were quantified using Simoa technology, while MRI data was analyzed via FreeSurfer to measure volumes of grey and white matter, specific brain structures, and ventricular sizes. Cognitive performance was assessed using the Symbol Digit Modalities Test (SDMT) and Brief Visuospatial Memory Test-Revised (BVMT-R). Disability was evaluated using the Expanded Disability Status Scale (EDSS). Results: The results indicated significant positive correlations between SDMT scores and volumes of grey matter, white matter, and various subcortical structures, suggesting that preserved brain volume is linked to better cognitive performance. Negative correlations were observed between SDMT scores and ventricular volumes, as well as between SDMT scores and EDSS scores, implying that cognitive decline corresponds with structural brain deterioration and increased disability. No significant associations were found between BVMT-R scores and imaging data or disability measures. Plasma NfL levels showed significant correlations with early disease relapses and enlargement of the third and fourth ventricles, but not with brain volume, cognitive tests, or EDSS scores. Conclusions: These findings indicate that MRI-based brain volumetrics, particularly grey and white matter measures, are stronger indicators of cognitive function and disability in RRMS than plasma NfL.