Background: The Cohesin complex plays a critical role in mitotic progression and post-replicative DNA damage repair. It serves to bring together sister chromatids both in metaphase and in homologous recombination repair following ionizing radiation. The complex has also been shown to be phosphorylated in the ATM/BRCA1 pathway. The expression of various proteins in the complex are dysregulated in many cancers: breast, prostate, etc. Interestingly, in breast cancer cell lines, Cohesin is required for MYC activation in response to estrogen. Our study sought to correlate copy number alterations in this pivotal complex with biochemical relapse in prostate cancer patients. Methods: Our cohort consists of 284 patients with D’ Amico-classified intermediate-risk prostate cancer, treated with image-guided radiotherapy (IGRT, N = 143) or radical prostatectomy (RadP, N = 141). Pre-treatment biopsies and prostatectomy samples were analyzed using the Affymetrix Oncoscan array. The Phoenix and AUA criteria was used to define biochemical relapse for RadP and IGRT patients respectively. Results: Copy number alterations of RAD21, SMC1B, and STAG1 were observed in 18% (n = 52), 6.3% (n = 18), and 12% (n = 35) of the cohort respectively. They were predominantly losses in SMC1B, but gains in RAD21 and STAG1. All three genes in the Cohesin complex were associated with increased risk of biochemical relapse: RAD21 on chromosome 8 (HR = 1.93, 95% CI 1.23, 3.02, Wald’s p = 0.004), SMC1B on chromosome 22 (HR = 3.37, 95% CI 1.91, 5.94, Wald’s p < 10^-4), and STAG1 on chromosome 3 (HR = 1.74, 95% CI 1.04, 2.89, Wald’s p < 0.05). However, when controlled for percent genome alteration and pre-treatment serum PSA levels, only copy number loss of SMC1B was a significant predictor of biochemical relapse (HR = 2.95, 95% CI 1.62, 5.38, Wald’s p < 10^-3). Conclusions: We identified a novel association of copy-number alterations in members of the Cohesin complex with biochemical recurrence following radical prostatectomy or image-guided radiotherapy. This points to the central role of Cohesin in cell-cycle and DNA damage pathways promoting prostate cancer progression.