"This thing is working," Obama said at an afternoon news conference. Of the GOP, he added: "They said no one would sign up. They were wrong about that. They are wrong to try to repeal a law that is working."
The final figure is well above the White House's initial target of 7 million signups.
Republicans, who have fought the law since it was passed by a sharply divided Congress in 2010, escalated their call for the law to be repealed after the problems with the enrollment Web site, which repeatedly broke down in its first few months. Obama's job approval ratings dropped, and Democrats worried that the bad headlines would harm the party at the ballot box.
But a buoyant Obama said the better-than-expected enrollment news should convince Democrats to not shy away from embracing his signature domestic achievement.
"I don't think we should apologize for it, and I don't think we should be defensive about it," he said. "I think it is a strong, good, right story to tell. I think what the other side is doing and what the other side is offering would strip away protections for those families."
Twenty-eight percent of those who enrolled are between 18 and 34, according to a White House fact sheet, below the target of 38 percent. But the administration has said that premiums are expected to cost 15 percent less than originally projected.
Despite the revised figures, the GOP is likely to continue to attack the law, which is unpopular among conservatives after years of political fighting over the administration's bid to expand health coverage for the uninsured.
Reacting to Obama's announcement, House Speaker John A. Boehner's office said the White House "continues to obscure the full impact" of the law, pointing to hundreds of thousands of people who had their insurance plans canceled and were forced to re-enroll in a new plan.
"The president ignores the havoc that this law has wreaked on private plans that people already had and liked," said Brendan Buck, spokesman for Boehner (R-Ohio).
During his remarks, Obama said he was "still puzzled why they've made this their sole agenda item when it comes to politics. It's curious."
Kathleen Sebelius resigned last week as secretary of Health and Human Services, the agency responsible for the rollout.