Background: Speckle-type POZ protein (SPOP) mutations are a frequent initiating event in prostate cancer (PC) and are associated with increased androgen receptor transcriptional output. While SPOP-Mutant (MT) PCs may be more sensitive to hormonal therapy, they are still common amongst castration-resistant PCs (CRPCs) and frequently lethal. It remains unknown which SPOP^MT PCs will respond well to hormonal therapy, and which have the potential to develop early castration resistance and cause patient death. Methods: With IRB approval, we retrospectively analyzed NGS data obtained via Tempus|xT tissue assay (DNA sequencing of 648 genes in tumor and matched normal samples at 500x depth) and/or Tempus|xF liquid biopsy assay (ctDNA sequencing of 105 genes in peripheral blood samples at 5,000x depth) for germline and/or somatic mutations in 234 consecutive patients with high-risk localized, locally advanced or metastatic PC followed at Ben Taub Hospital (BTH), a public hospital serving a racially/ethnically diverse patient population (>50% of PC patients are African American). For confirmation, we examined de-identified NGS data from a nationwide cohort of 8812 PC patients sequenced with xT and/or xF by Tempus Labs (Chicago, IL), as well as publicly available data from four additional PC cohorts (PMIDs: 26544944, 34667026, 31061129 and 26855148). Results: We found SPOP mutations in 31/234 BTH PC patients (13.2%). Eleven of these 31 patients (35.5%) also exhibited loss-of-function (LOF) mutations or deletions in the tumor suppressor gene Adenomatous polyposis coli (APC). APC LOF events were found only in 15/203 (7.4%) of the SPOP-wild type (WT) PCs, and the enrichment of SPOP^MT PCs for APC LOF events was highly significant (Chi square with Yates correction: P<0.00001). The co-mutation pattern between these two genes was observed even when the analysis was limited specifically to the tissue NGS (133 patients) or the liquid biopsy NGS subsets of the dataset, respectively. This enrichment was confirmed in the nationwide TEMPUS cohort and in the four publicly available clinical PC datasets (APC LOF found in 23.4, 27.0, 21.5, 32.0 and 30.8% of SPOP^MT PCs, respectively; and only in 5.8, 2.4, 6.0, 6.7 and 1.0% of SPOP-Wild Type (WT) PCs, respectively (P<0.001 in each cohort). In the BTH PC cohort, SPOP^MT PCs harboring APC LOF mutations were more likely to present with metastatic disease (M1) at diagnosis and to develop castration-resistance within 1 year of first-line hormonal therapy than SPOP^MT PCs with WT APC. Conclusions: In SPOP^MT PCs, inactivation of APC is a common event and associated with more aggressive disease (higher metastatic potential and faster progression to castration resistance). This observation may help identify SPOP^MT PC patients who are at higher risk for early castration resistance and could benefit from more intensive treatment.