5-10% of total prostate cancer (PCa) cases are hereditary. Particularly, immune checkpoint inhibitor-sensitive tandem duplicator phenotype (TDP) accounts for 6.9% of PCa cases, whereas genetic susceptibility genes remain completely unknown. We identified a Chinese family with two PCa patients, in which the PCa phenotype co-segregated with a rare germline variant EGFRR831H. Patient-derived conditionally reprogrammed cells (CRC) exhibited increased EGFR and AKT phosphorylation, and a sensitivity to EGFR antagonist Afatinib in migration assays, suggesting the EGFR allele was constitutively active. Both EGFRR831H-mutant tumours contained biallelic CDK12 inactivation, together with prominent tandem duplication across the genome. These somatic mutations could be detected in urine before surgery. Analysis of public databases showed a significant correlation between the mutation status of EGFR and CDK12. Taken together, our genetic and functional analyses identified a previously undescribed link between EGFR and PCa.