IMPORTANCE: Fragmentation between Medicare and Medicaid exacerbates access barriers, delays in care, and poor financial and health outcomes among the more than 4 million dually eligible beneficiaries with serious mental illness (SMI). Integrated dual-eligible special needs plans (D-SNPs), including highly integrated D-SNPs (HIDE-SNPs) and fully integrated D-SNPs (FIDE-SNPs), are Medicare Advantage (MA) plans that financially integrate Medicare and Medicaid benefits and may address the significant behavioral health, medical, and long-term service and support needs in this population. OBJECTIVE: To examine enrollment and disenrollment in integrated D-SNPs relative to other Medicare options among dually eligible beneficiaries with SMI from 2019 to 2022. DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study of a 20% sample of dually eligible beneficiaries with SMI and full Medicaid benefits used Medicare administrative data from 2019 to 2022, including traditional Medicare fee-for-service claims data and MA encounter data. Data were analyzed from September 1, 2025, to April 3, 2026. MAIN OUTCOMES AND MEASURES: Enrollment and disenrollment in integrated D-SNPs relative to other Medicare options (traditional Medicare, standard MA, and coordination-only D-SNPs) from 2019 to 2022, as well as enrollee demographic, clinical, and geographic characteristics. RESULTS: This study included 719 542 dually eligible beneficiaries with a diagnosis of SMI (mean [SD] age, 64 [16] years; 459 555 women [63.9%]). The proportion of dually eligible beneficiaries with SMI enrolled in integrated HIDE-SNPs or FIDE-SNPs increased from 2.4% in 2019 to 17.2% in 2022, with a corresponding decrease in traditional Medicare enrollment from 58.6% to 42.6% in the same period. Cardiovascular disease (CVD), which is associated with excess mortality in SMI, was prevalent: 32.1% of beneficiaries had 3 CVD risk factors. More than half of states had no integrated D-SNP offerings, including states ranked highest for prevalence of comorbid CVD among dually eligible beneficiaries with SMI. A total of 10.3% of dually eligible beneficiaries with SMI enrolled in HIDE-SNPs and 6.1% of beneficiaries enrolled in FIDE-SNPs in 2021 disenrolled by 2022, relative to 20.9% disenrollment from standard MA. CONCLUSIONS AND RELEVANCE: This retrospective cohort study of dually eligible beneficiaries with SMI found increases in integrated D-SNP enrollment and limited disenrollment, which may suggest a high level of enrollee satisfaction. More research is needed to assess the association of integrated D-SNP enrollment with health outcomes among dually eligible beneficiaries with SMI.