Importance: People with schizophrenia are at high risk of receiving a diagnosis of dementia. Understanding the magnitude and timing of this increased risk has important implications for practice and policy. Objective: To estimate the age-specific incidence and prevalence of dementia diagnoses among older US adults with schizophrenia and in a comparison group without serious mental illness (SMI). Design, Setting, and Participants: This retrospective cohort study used a 50% random national sample of Medicare beneficiaries 66 years or older with fee-for-service plans and Part D prescription drug coverage from January 1, 2007, to December 31, 2017. The cohort with schizophrenia included adults with at least 12 months of continuous enrollment in fee-for-service Medicare and Part D and at least 2 outpatient claims or at least 1 inpatient claim for schizophrenia during the qualifying years. The comparison group included adults with at least 12 months of continuous enrollment in fee-for-service Medicare and Part D and without a diagnosis of schizophrenia, bipolar disorder, or recurrent major depressive disorder during the qualifying year. Data were analyzed from January 1 to July 31, 2020. Main Outcomes and Measures: Dementia was defined using the Centers for Medicare & Medicaid Services Chronic Conditions Warehouse diagnosis codes for Alzheimer disease and related disorders or senile dementia. Incident diagnoses were defined by at least 12 consecutive eligible months without a qualifying code before meeting dementia criteria. Results: The study population of 8 011 773 adults 66 years or older (63.4% women; mean [SD] age, 74.0 [8.2] years) included 74170 individuals with a diagnosis of schizophrenia (56.6% women) and 7937603 without an SMI diagnosis (63.5% women) who contributed 336814 and 55499543 person-years of follow-up, respectively. At 66 years of age, the prevalence of diagnosed dementia was 27.9% (17 640 of 63 287) among individuals with schizophrenia compared with 1.3% (31 295 of 2 389 512) in the group without SMI. By 80 years of age, the prevalence of dementia diagnoses was 70.2% (2011 of 2866) in the group with schizophrenia and 11.3% (242 094 of 2 134 602) in the group without SMI. The annual incidence of dementia diagnoses per 1000 person-years at 66 years of age was 52.5 (95% CI, 50.1-54.9) among individuals with schizophrenia and 4.5 (95% CI, 4.4-4.6) among individuals without SMI and increased to 216.2 (95% CI, 179.9-252.6) and 32.3 (95% CI, 32.0-32.6), respectively, by 80 years of age. Conclusions and Relevance: In this cohort study, compared with older adults without SMI, those with schizophrenia had increased risk of receiving a diagnosis of dementia across a wide age range, possibly because of cognitive and functional deterioration related to schizophrenia or factors contributing to other types of dementia. High rates of dementia among adults with schizophrenia have implications for the course of illness, treatment, and service use.