Abstract Over 70 percent of older adults aged 65 or older require assistance from caregivers with basic functions at some point in their lifetime, and most of that care is provided informally by families and friends. These informal caregivers, especially those to older adults with Alzheimer’s disease and Alzheimer’s disease-related dementias (AD/ADRD), bear huge physical, phycological, and financial costs which limit their ability to provide care. The most recent Medicaid expansion under the Patient Protection and Affordable Care Act (ACA-ME) has been shown to substantially increase health insurance coverage and health care utilization, improve enrollees’ physical and mental health and financial security, and influence their labor market outcomes. All these effects could be potential mechanisms through which the ACA-ME impacts informal care. However, despite the large number of studies on ACA-ME, little is known about its effects on informal care for older adults. We propose to fill this gap by examining the effects of the ACA-ME on informal care provided to and received by older adults. Specifically, we will use a difference-in-differences approach and compare informal care provision and receipt among individuals living in states that expanded their Medicaid program before and after the expansion relative to the change in informal care provision and receipt experienced by comparable individuals living in non-expansion states over the same period. We will use the American Time Use Survey to measure informal care provided by low-income adults aged 18 to 64 who are most likely to be affected by the ACA-ME. We will use the Health Retirement Study to measure informal care received by older adults. By considering this spillover effect of the ACA-ME on informal care, the proposed project will be important for policymakers interested in designing more effective policies to sustain or even increase the supply of informal care to meet the escalating demand and for those interested in a more comprehensive evaluation of the ACA- ME.