OBJECTIVE: To characterize the landscape of policies that determine eligibility for family planning services through Medicaid programs and describe trends in eligibility and its determinants over time. DATA SOURCES AND STUDY SETTING: Secondary data were collected for all states in the United States for the years 2008 through 2023. Data on economic and demographic characteristics came from the American Community Survey (ACS). STUDY DESIGN: Our descriptive study characterized state adoptions of Medicaid family planning section 1115 waivers and state plan amendments (SPA) and their eligibility criteria. We then estimated the proportion of women aged 19-44 years who were eligible for family planning services through Medicaid and identified the key determinants of changes in eligibility, by state and year. DATA COLLECTION/EXTRACTION METHODS: Information on state Medicaid policies was extracted from documentation on the Centers for Medicare & Medicaid Services website. When estimating the eligible population sizes, the denominator was women aged 19-44 years, the group most likely to be eligible for Medicaid family planning programs. Supplemental data on program enrollment or utilization were collected from states' websites and reports. PRINCIPAL FINDINGS: Though eligibility limits for family planning through Medicaid generally increased over time, the proportion of women aged 19-44 years eligible for at least limited benefits decreased from 45.0% in 2012 to 39.4% in 2022, largely because of increases in household income. Trends varied considerably across states and by eligibility pathway. Among women with incomes below the poverty level, the proportion who were not eligible for Medicaid family planning services decreased from 6.3% in 2013 to 1.5% in 2022. CONCLUSIONS: Our data demonstrated substantial geographic and temporal variation in eligibility for family planning services through Medicaid. We identified key drivers of eligibility changes that may have important implications for health services analyses of means-tested public programs.