While high doses of estrogen, in combination with androgens, can initiate prostate neoplasia via activation of the estrogen receptor α (ERα), the role of ERα in prostate cancer cells within established tumors is largely unknown. Here we show that expression of ERα is increased in high grade human prostate cancer. Similarly, ERα is elevated in mouse models of aggressive prostate cancer driven by the overexpression of the proto-oncogene MYC or the conditional deletion of the tumor suppressor PTEN. Furthermore, within the prostate of PTEN-deficient mice, there is a progressive pattern of ERα expression: low in benign glands, moderate in tumors within the dorsal, lateral and ventral lobes, and high in tumors within the anterior prostate. This expression significantly correlates with the levels of the proliferation marker Ki67 and in vitro knockdown of ERα in cells derived from PTEN-deficient tumors causes a significant and sustained decrease in proliferation. Depletion of ERα also reduces the levels of the MYC protein. Finally, ERα knockdown reduces the activity of the PI3K and MAPK pathways, which are both downstream targets of non-genomic ERα action. Collectively, these results demonstrate that ERα orchestrates cell proliferation and is a key driver of the neoplastic growth of prostate cancer cells.