Background. New therapies for castration-resistant prostate cancer (CRPC) may be associated with increased risk of second primary malignancies (SPM). We therefore estimated the population-based incidence of SPM among patients with CRPC in the Surveillance, Epidemiology, and End Results (SEER)-Medicare database. We also estimated the proportion of men with CRPC with bone metastases and overall survival. Methods. We conducted a retrospective cohort study of United States (US) men aged ≥ 65 years with CRPC. Cohort entry was from January 1, 2000, to December 31, 2011, with follow-up through December 31, 2013. Castration resistance was defined by treatment with second-line systemic therapy (after surgical or medical castration). SPM were diagnoses of primary cancers (other than prostate) in SEER or Medicare data. Results. Altogether 2,234 patients met eligibility criteria. Most (1,887; 84.5%) had evidence of bone metastases in Medicare claims. SPM occurred in 172 patients (incidence rate 5.9 per 100 person-years; 95% confidence interval [CI], 5.0-6.8; standardized incidence ratio = 3.1, 95% CI, 2.8-3.6, based on SEER incidence rate of all malignancies except prostate cancer among men aged ≥ 65 years). The most common SPM were lung/bronchus (n = 29, 16.9%), urinary bladder (n = 22, 12.8%), and colon/rectum (n = 21, 12.2%). Median survival was 1.2 years (95% CI, 1.1-1.3); 5-year survival was 9% (95% CI, 7-11%). Conclusions. This study provides the first estimate of SPM risk in older men with CRPC in the US. The incidence rate is approximately threefold higher than the population-based cancer incidence among men without prostate cancer.