Abstract Excess sarcoma risks after childhood cancer are well established, but risks among young adulthood cancer survivors are poorly understood. Using US population-based cancer registry data, we compared bone and soft-tissue sarcoma risk vs the general population among 186 351 individuals who were diagnosed with nonsarcoma first primary malignancies at ages 20–39 years from 1975 to 2014 (follow-up through 2015) and survived at least 1 year. Bone sarcomas were rare (n = 50), but risk was statistically significantly elevated overall (2.9-fold) and greater than fivefold after Hodgkin lymphoma, non-Hodgkin lymphoma, and central nervous system tumors. Soft-tissue sarcomas were more common (n = 284) and risks were statistically significantly elevated approximately twofold overall and after melanoma and carcinomas of the breast, thyroid, and testis, and greater than fourfold after Hodgkin lymphoma and central nervous system tumors. Risks varied markedly by subtype, with the highest risks (greater than fourfold) for osteosarcoma and the soft-tissue subtypes of rhabdomyosarcoma and blood vessel and nerve sheath sarcomas. These data demonstrate elevated risk for sarcoma after a range of young adulthood cancers.