<p>Neurodegenerative diseases (NDDs) exhibit considerable molecular heterogeneity, making it difficult to pinpoint robust, disease-specific biomarkers. Although proteomic studies have deepened our understanding of individual disorders, systematic cross-disease comparisons with cross-platform validation remain scarce, especially for rare conditions like spinal and bulbar muscular atrophy (SBMA). To address this gap, we conducted a comparative plasma proteomic analysis using liquid chromatography–tandem mass spectrometry (LC–MS/MS) in 264 participants across major neurodegenerative and related diagnostic groups, including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), SBMA, and cognitively healthy controls. This unified framework allowed us to capture both disease-specific and shared protein signatures across neurodegenerative conditions. Candidate proteins were then validated in the UK Biobank (Olink Explore) and the Global Neurodegeneration Proteomics Consortium (SomaScan). Of 23 proteins assessed in the UK Biobank, four unique proteins (yielding six disease–protein associations) showed nominally significant and directionally concordant changes; of 20 proteins represented by 27 probes tested in the Global Neurodegeneration Proteomics Consortium, seven proteins reached nominal significance, all with full directional concordance across both cohorts. Notably, IGFBP2 was consistently elevated in AD and PD across independent datasets, pointing to shared metabolic dysregulation, while ADIPOQ showed parallel increases in the same conditions, reinforcing convergent shifts in energy metabolism. By contrast, CRTAC1 and COMP were selectively reduced in motor neuron diseases, suggesting disease-enriched alterations in extracellular matrix composition. Taken together, our findings provide a cross-disease, cross-platform framework for uncovering reproducible proteomic biomarkers and shed light on both overlapping and distinct molecular pathways in neurodegeneration.</p>