The total, which includes 1.1 million people who enrolled in April alone, does not indicate specifically how many people have gained coverage through the Affordable Care Act's expansion of Medicaid, a program run by states but overseen by the federal government.
But the U.S. Centers for Medicare and Medicaid Services said enrollment in states that have expanded Medicaid grew much faster than in other states -- 15.3 percent, vs. 3.3 percent in states that have not expanded the program.
Enrollment figures including existing Medicaid programs and the Children's Health Insurance Program, which provides coverage for children from families who earn too much to qualify for Medicaid but still cannot afford private coverage.
President Barack Obama's signature domestic policy, the law known as Obamacare, was designed to extend health coverage to millions of uninsured Americans through subsidized private health insurance and by expanding Medicaid to people with incomes of up to 133 percent of the federal poverty level, or $15,521 for an individual and $31,721 for a family of four.
More than 8 million people have enrolled in private health insurance through Obamacare marketplaces set up in all 50 states, according to administration officials.
In 2012, the U.S. Supreme Court allowed states to opt out of the Medicaid expansion. Since then, about half of states have moved to expand coverage to the poor while another four are debating the possibility. The remainder have not moved forward, often because of Republican opposition to Obamacare.