Background <p>Colorectal cancer (CRC) remains a leading cause of cancer-related morbidity and mortality. Although Synaptotagmin 8 (SYT8) has been implicated in tumorigenesis, its role in CRC is poorly understood.</p> Methods <p>Transcriptomic and clinical data from TCGA and five GEO datasets were analyzed, and survival analysis together with nomogram modeling was applied to evaluate the prognostic value of SYT8. Functional enrichment and immune infiltration analyses were performed to investigate its potential roles and its impact on the tumor microenvironment. SYT8 expression in CRC and adjacent normal tissues was validated by qRT-PCR and western blotting. Knocking down experiments in vitro were conducted to assess its effects on cell proliferation, migration, and related signaling pathways.</p> Results <p>Compared to normal samples, SYT8 was significantly highly expressed in CRC tissues. Additionally, elevated SYT8 expression was strongly associated with poorer overall survival and identified as an independent prognostic factor. Functional enrichment analysis indicated its potential involvement in multiple signaling pathways, including MAPK and PI3K–Akt. Immune-related analyses revealed that tumors with high SYT8 expression exhibited increased Treg infiltration and upregulation of immune checkpoint molecules PDCD1 and LAG3. Significantly, downregulation of SYT8 suppressed HT29 cell viability, proliferation, and migration. Moreover, SYT8 downregulation notably reduced the phosphorylation of Akt and ERK1/2 in HT29 cells.</p> Conclusions <p>SYT8 plays a critical role in the development and progression of CRC and holds potential as a prognostic biomarker, as well as a novel therapeutic target.</p>