OBJECTIVES: Medigap protects traditional Medicare (TM) beneficiaries against catastrophic expenses. Federal regulations around Medigap enrollment and pricing are limited to the first 6 months after turning 65 years old. Eight states institute regulations that apply to later enrollment; half use community rating (charging everyone the same premium) and half use both community rating and guaranteed issue (requiring insurers to accept any beneficiary irrespective of health conditions). We examined the impact of state-level Medigap regulations on insurance coverage and health care spending for Medicare beneficiaries. STUDY DESIGN: We used a retrospective cohort study design. Using the 2010-2016 Medicare Current Beneficiary Survey, we identified beneficiaries with TM only, TM + Medigap, or Medicare Advantage (MA) by state-level Medigap regulations. METHODS: Outcomes were insurance coverage and health care spending. We used an instrumental variable approach to address endogenous insurance choice. We conducted 2-stage least squares regression while controlling for individual-level characteristics and area-level demographic characteristics. Then we used the recycled prediction methods to predict enrollment and spending outcomes for the 3 state-level Medigap regulation scenarios. RESULTS: Although enrollment in TM only was consistent across regulation scenarios, the scenario with community rating and guaranteed issue had lower Medigap enrollment and higher MA enrollment than the no-regulation scenario. Despite negligible health differences, TM + Medigap beneficiaries had higher Medicare spending than TM-only beneficiaries, suggesting moral hazard. CONCLUSIONS: Our findings suggest a link between additional regulations and lower Medigap and higher MA enrollment. Policy makers should consider the potential effects on insurance coverage, premiums, financial protection, and moral hazard when designing Medigap regulations.