Register-based studies indicate a possible association of monoclonal gammopathy of undetermined significance (MGUS) and prostate cancer (PCa). Aim of the present study was to investigate the relationship between MGUS and PCa considering potentially shared risk factors. Data from the prospective population-based Heinz Nixdorf Recall cohort study of 2.385 men (age 45–85) were analyzed. MGUS was determined at three points in time; cases of cancer were assessed annually. Potentially shared risk factors were assessed at baseline. Hazard ratios (HR), adjusted for age and educational attainment, and corresponding 95%-confidence intervals (95%-CI) were calculated. 157 cases of MGUS and 143 incident cases of PCa were detected. Of 19 participants diagnosed with both, MGUS and incident PCa, only in one case MGUS did not clearly occur before PCa. MGUS was associated with PCa presenting a HR of 2.00 (95%-CI: 1.23–3.25). Stratified by isotype, IgM-MGUS showed the strongest association with PCa. There was no relevant change of the effect estimate when adjusting for potentially shared risk factors. We were able to give supporting evidence for an association between MGUS and PCa and pointed out its temporality. There was no indication that the observed association is due to shared risk factors. The present study indicated that different isotypes of MGUS differ in the strength of the effect on PCa-risk. Based on these findings, future studies investigating the pathophysiological background of the association will be needed.