WASHINGTON — About 12.7 million people signed up for health insurance under the Affordable Care Act or had their coverage automatically renewed in the third annual open enrollment season, the Obama administration said Thursday.

Sylvia Mathews Burwell, the secretary of the Department of Health and Human Services, said the signups exceeded her goals and her expectations. “Open enrollment for 2016 is over, and we are happy to report it was a success,” she said.

Most of the plan selections were for people in the 38 states — more than 9.6 million — who used the federal website, HealthCare.gov. The other 3.1 million people were enrolled in states that run their own marketplaces.

Ms. Burwell said she was pleased to see that four million of the people signing up through HealthCare.gov were new to the federal insurance marketplace in 2016. This, she said, shows that “marketplace coverage is a product people want and need.”