Solitary confinement is a severe form of incarceration closely associated with long-lasting psychological harm and poor post-release outcomes. Estimating the population prevalence, we find that 11% of all black men in Pennsylvania, born 1986 to 1989, were incarcerated in solitary confinement by age 32. Reflecting large racial disparities, the population prevalence is only 3.4% for Latinos and 1.4% for white men. About 9% of black men in the state cohort were held in solitary for more than 15 consecutive days, violating the United Nations standards for minimum treatment of incarcerated people. Nearly 1 in 100 black men experienced solitary for a year or longer by age 32. Racial disparities are similar for women, but rates are lower. A decomposition shows that black men’s high risk of solitary confinement stems primarily from their high imprisonment rate. Findings suggest that harsh conditions of U.S. incarceration have population-level effects on black men’s well-being.