There are currently more adults age fifty-five or older incarcerated in the United States than ever before. Little is known about the epidemiology or health care needs of geriatric patients in jails, where the majority of the population is being held in pretrial detention. We performed a retrospective analysis of electronic health record data to characterize the demographics, health conditions, and health care use of people age fifty-five or older who were incarcerated in the New York City jail system between 2015 and 2019. People in this age group accounted for 4 percent of admissions to the jails in 2009, ten years before the study's end date; 7 percent of admissions in 2015, when the study began; and 8.5 percent of admissions by 2019. They were more likely to report being homeless; suffer from a serious mental illness designation; carry a higher burden of chronic, infectious, and serious medical illnesses; be hospitalized during their incarceration; and die in jail custody than their younger counterparts. All elements of the criminal justice system need to be attuned to the vulnerabilities of this group, implement targeted interventions to divert them from incarceration when possible, and minimize harms for those who end up incarcerated.