Increased androgen receptor (AR) activity drives therapeutic resistance in advanced prostate cancer. The most common resistance mechanism is amplification of this locus presumably targeting the AR gene. Here, we identify and characterize a somatically acquired AR enhancer located 650 kb centromeric to the AR. Systematic perturbation of this enhancer using genome editing decreased proliferation by suppressing AR levels. Insertion of an additional copy of this region sufficed to increase proliferation under low androgen conditions and to decrease sensitivity to enzalutamide. Epigenetic data generated in localized prostate tumors and benign specimens support the notion that this region is a developmental enhancer. Collectively, these observations underscore the importance of epigenomic profiling in primary specimens and the value of deploying genome editing to functionally characterize noncoding elements. More broadly, this work identifies a therapeutic vulnerability for targeting the AR and emphasizes the importance of regulatory elements as highly recurrent oncogenic drivers. Display omitted •An AR enhancer becomes activated in castrate-resistant prostate cancer (CRPC)•The AR enhancer is frequently amplified in CRPC•The enhancer amplification desensitizes cancer cells to hormone deprivation treatment•The AR enhancer is likely to be a developmental enhancer that is reactivated in CRPC Activation and amplification of an enhancer upstream of the androgen receptor locus drives progression of metastatic castration-resistant prostate cancer.