Our previous studies have found that activation of Wnt/β-catenin signaling resulted in mouse prostatic intraepithelial neoplasia (mPIN). In the large probasin promoter directed SV40-large T-antigen (LPB–Tag) expressing mouse prostate, mPIN forms with rare areas of adenocarcinoma. Combining expression of both Wnt-signaling and Tag expression in the mouse prostate, we have studied the role of Wnt/β-catenin signaling in the progression from mPIN to adenocarcinoma. Our results show that the prostates of mice expressing Tag alone or nuclear β-catenin alone developed mPIN, whereas the activation of both Tag and the Wnt/β-catenin pathway resulted in invasive prostate adenocarcinoma. Furthermore, Foxa2, a forkhead transcription factor, was induced by active Wnt/β-catenin signaling, and the expression of Foxa2 was associated with the invasive phenotype in the primary prostate cancer. In the LPB–Tag/dominant active (DA) β-catenin prostates, MMP7, a Wnt/β-catenin target gene, was upregulated. Furthermore, we also assessed AR and AR signaling pathway in these LPB–Tag/DA β-catenin mice. Although β-catenin is a well-known AR co-activator in vitro, our study provides strong in vivo evidences indicating that both AR protein and the AR pathway were downregulated in the prostate of LPB–Tag/DA β-catenin mice. Histological analysis shows that prostate sections derived from the LPB–Tag/DA β-catenin mice display neuroendocrine differentiation (NED), but NE cancer does not develop. Together, our findings indicate that Wnt/β-catenin signaling has an important role in the progression of mPIN to prostate adenocarcinoma.