Background: Prostate cancer (PCa) is a significant public health issue, particularly in developed countries. The androgen receptor (AR) plays a key role in regulating both the normal development and the proliferation of PCa. Bipolar androgen therapy, which involves treatment with supraphysiological androgen levels (SALs), has been shown to inhibit PCa growth. SAL induces cellular senescence in AR-positive PCa cell lines, human tumor samples, and xenografted mouse models. Methods: Transcriptome and chromatin immunoprecipitation (ChIP)-seq analysis, ChIP-qPCR, knockdown (KD), overexpression (OE), qRT-PCR, immunodetection, in situ histochemistry. Results: Here, we show using ChIP-seq and RNA-seq that H2AJ, a variant of the canonical histone H2A, is a direct target gene of AR that regulates cellular senescence and the formation of senescence-associated heterochromatin foci (SAHF). Accordingly, bioinformatic analyses reveal a large overlap of the H2AJ transcriptome with the cellular senescence score of PCa. Analyzing a large cohort of patient samples, the expression of H2AJ is higher in tumor samples compared to normal samples suggesting growth-promoting activity. Interestingly, however, the expression is diminished in metastatic tumor samples, indicating that H2AJ inhibits the mesenchymal transition in PCa cells. Functionally, the KD of H2AJ inhibits growth, whereas the H2AJ overexpression promotes cell growth. Furthermore, these data suggest that H2AJ inhibits the expression of mesenchymal markers, in agreement with the low expression of H2AJ in metastatic forms of tumors from patient cohorts. Conclusion: H2AJ is a direct positively AR-regulated target gene induced by SALs that regulates cellular senescence, promotes growth, and inhibits the expression of mesenchymal markers.