Sporadic amyotrophic lateral sclerosis (sALS) is a fatal adult-onset neurodegenerative disease with unknown causes and few treatment options. This study employed single-cell RNA sequencing and immune profiling to analyze peripheral blood mononuclear cell samples from sALS patients and depicted the first cellular landscape at single-cell resolution. Compared to healthy controls, ALS patients exhibit markedly reduced naïve T cell and B cell populations, with significant clonal expansion of terminal effector T cells. However, no common recognition patterns were detected among CDR3 regions of expanded T cell receptor clonotypes, suggesting that each ALS patient may have been triggered by different antigens. We discovered a novel CD8+ T cell type with heterogenous expression profiles of stemness genes and carrying signature genes specific for monocytes and terminal effector T cells, which may play a critical role in ALS pathogenesis. Using differentially expressed genes between the ALS and Control groups within the novel CD8+ T cell type, we identified 80 genes that collectively distinguished ALS patients from healthy controls with an unprecedented accuracy of 0.929 (sensitivity: 0.928, specificity: 0.930). Our findings from this study have important implications for understanding the pathogenesis of sALS, and provide new strategies for accurate diagnosis and potential therapeutic development.SummarySingle-cell RNA sequencing and immune profiling analysis of ALS patient PBMCs provided the first cellular landscape at single-cell resolution, unraveled possible causes and improved diagnosis.