Adolescent and young adult (AYA) cancer incidence rates are rising, and survivors are at risk for numerous cancer- and treatment-related consequences. Despite growing attention to this population, prevalence estimates are lacking. To estimate the number of individuals living in the United States with a history of cancer diagnosed during the AYA period. Prevalence of cancer survivors diagnosed between the ages of 15 and 39 years was estimated using data from the Surveillance, Epidemiology, and End Results (SEER) program as of January 1, 2020. Limited duration prevalence data were also used to generate complete prevalence by sex, years since diagnosis (0-<1, 1-<5, 5-<10, 10-<15, 15-<20, 20+), and attained age (15-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70+) for the 15 most common AYA cancer sites. There were an estimated 2,111,838 survivors of AYA cancers in the United States as of January 1, 2020. More survivors were female (66%) and long-term (>5 years from diagnosis, 83%) or very long-term survivors (>10 years from diagnosis, 68.8%). A large percentage (44%) were more than 20 years from diagnosis. The most common cancer sites among females were breast (24%) and thyroid cancers (23%) and, among males, testicular cancer (31%). Across the population, the highest percentage of survivors of AYA cancers were 40- to 49-years of age (25.3%). There are over 2.1 million cancer survivors diagnosed in the AYA period who are living in the United States; most are more than 10 years from diagnosis.