The robust enrolment pace defies President Donald Trump's insistence that the Affordable Care Act or Obamacare is "dead" and will likely make Republican efforts to dismantle the 2010 healthcare law even more challenging after failed repeal efforts this year, Politico magazine reported.
The number is slightly lower than the last year when 9.2 million people signed up for coverage during an open enrolment period that was twice as long.
Approximately 8.8 million people enrolled during the six-week open enrolment period in the 39 states that use the federal healthcare.gov website, the Centres for Medicare and Medicaid Services (CMS) said.
Obamacare supporters said sign-ups far surpassed their expectations, given the ongoing repeal effort in Congress, Trump's harsh rhetoric against the law and a series of actions taken by his administration seen as undermining enrolment.
The Trump administration halved this year's enrolment period to six weeks, slashed advertising by 90 per cent and cut funding for in-person enrolment aides by over 40 per cent.
"This is a remarkable result given the administration's efforts to sabotage enrolment by gutting outreach, creating chaos and confusion, cutting off subsidies for low-income families and shortening the enrolment period by six weeks," said Representative Frank Pallone, the top Democrat on the House Energy and Commerce Committee.
"Today's enrolment numbers make clear that the American people want access to high quality, affordable health insurance coverage and they want Congress and the Administration to stop playing games with our healthcare system," he added.
In a tweet, CMS administrator Seema Verma said her agency had done a great job to "make this the smoothest experience for consumers to date".
Joshua Peck, who was the chief marketing officer for HealthCare.gov in the Obama administration, said: "It's a very, very strong number and implies that the final week of open enrolment this year was very big."
The final enrolment numbers are expected to grow and could surpass the 12 million total people who signed up last year.