Although most non-elderly adults have health insurance through their own or through a family member’s employer, gay and lesbian adults face barriers to adding their partners to employer-sponsored health plans. When states adopt same-sex marriage, “fully-insured” employers regulated by state insurance laws are required to extend health benefits offered to opposite-sex spouses of employees to same-sex partners of employees. This study takes advantage of the “natural experiment” that occurred in the four study states enacting same-sex marriage between 2008 and 2011 (Connecticut, Iowa, New Hampshire and Vermont) to measure the impact of same-sex marriage on health insurance coverage among same-sex couples. We use a difference-in-differences approach to compare health insurance coverage among targeted same-sex couples before and after each state implements legal same-sex marriage. We rely on data from the 2008-2011 American Community Surveys to identify non-elderly adults (25-64 years) in same-sex relationships affected by new same-sex marriage provisions (n=1,249) and two comparison groups:  married opposite-sex couples unaffected by state policy changes (n=122,576) and same-sex couples in similar states not adopting same-sex marriage (n=31,107). Adjusted models include controls for age, education, income, employment stats, industry, citizenship, and the presence of an own child in the household. Unadjusted health insurance coverage increased 6% for women and negligibly for men in same-sex relationships following the implementation of same-sex marriage across the four study states. After controlling for demographic characteristics in the difference-in-differences models, same-sex marriage led to an adjusted 6-8% (p<0.05) increase in health insurance coverage for women in same-sex relationships; a similar effect was not found for men in same-sex relationships. This study provides early evidence that legal same-sex marriage expands health benefits as its advocates suggest. Gender-specific differences in our results also suggest that lesbian couples may potentially gain more economic and health benefits than male gay couples following the adoption of same-sex marriage.