Medicare Advantage now covers twenty-eight million older adults, many of whom have mental health needs. Enrollees are often restricted to providers who participate in a health plan's network, which may present a barrier to care. We used a novel data set linking network service areas, plans, and providers to compare psychiatrist network breadth-the percentage of providers in a given area that are considered "in network" for a plan-across Medicare Advantage, Medicaid managed care, and Affordable Care Act plan markets. We found that nearly two-thirds of psychiatrist networks in Medicare Advantage were narrow (that is, they contained fewer than 25 percent of providers in a network's service area) compared with approximately 40 percent in Medicaid managed care and Affordable Care Act plan markets. We did not observe similar differences in network breadth for primary care physicians or other physician specialists across markets. Amid efforts to strengthen network adequacy, our findings suggest that psychiatrist networks in Medicare Advantage are particularly narrow, which may disadvantage enrollees as they attempt to obtain mental health services.