Polyglutamine (polyQ) tract polymorphism within the human androgen receptor (AR) shows population heterogeneity. African American men possess short polyQ tracts significantly more frequently than Caucasian American men. The length of polyQ tracts is inversely correlated with the risk of prostate cancer, age of onset, and aggressiveness at diagnosis. Aberrant activation of Wnt signaling also reveals frequently in advanced prostate cancer, and an enrichment of androgen and Wnt signaling activation has been observed in African American patients. Here, we assessed aberrant expression of AR bearing different polyQ tracts and stabilized β-catenin in prostate tumorigenesis using newly generated mouse models. We observed an early onset oncogenic transformation, accelerated tumor cell growth, and aggressive tumor phenotypes in the compound mice bearing short polyQ tract AR and stabilized β-catenin. RNA sequencing analysis showed a robust enrichment of Myc-regulated downstream genes in tumor samples bearing short polyQ AR versus those with longer polyQ tract AR. Upstream regulator analysis further identified Myc as the top candidate of transcriptional regulators in tumor cells from the above mouse samples with short polyQ tract AR and β-catenin. Chromatin immunoprecipitation analyses revealed increased recruitment of β-catenin and AR on the c-Myc gene regulatory locus in the tumor tissues expressing stabilized β-catenin and shorter polyQ tract AR. These data demonstrate a promotional role of aberrant activation of Wnt/β-catenin in combination with short polyQ AR expression in prostate tumorigenesis and suggest a potential mechanism underlying aggressive prostatic tumor development, which has been frequently observed in African American patients.