FAM83B is a novel oncogene that mediates transformation. Despite emerging evidence supporting an association between FAM83B and cancer, a holistic view of FAM83B’s correlation with pan-cancer is limited and its carcinogenic and radioresistant roles in esophageal squamous cell carcinoma (ESCC) remain to be explored. Using data from the TCGA project, GTEx database, and other online resources, we comprehensively examined FAM83B expression, genetic mutation, copy number variations (CNV), methylation, prognosis, function, immune-associated analyses, and drug sensitivity in pan-cancer. In addition, the biological function of FAM83B in ESCC was verified by CCK-8, colony formation assays, and flow cytometry. We discovered aberrant expression of FAM83B affected prognosis in various malignant tumors. Abnormal FAM83B mRNA expression was associated with CNV and methylation. Significant correlations were also observed between FAM83B expression and immune cell infiltration, immune checkpoints, tumor mutational burden (TMB), and microsatellite instability (MSI) in malignancies. In vitro experiments indicated that FAM83B mRNA and protein were upregulated in ESCC, and knockdown of FAM83B significantly inhibited the proliferation while promoting apoptosis and radiosensitivity of ESCC. These results suggest the multiple functional roles of FAM83B in pan-cancer and provide an attractive diagnostic and therapeutic biomarker for certain cancer types, especially ESCC.