The official said Obama - on vacation with his family in Hawaii for the holidays - signed up over the weekend for "a health care plan made available by the Affordable Care Act on the DC marketplace".
The gesture was a "symbolic" one, the official said on condition of anonymity, as Obama receives health care from the military, as all presidents do. He also has a White House medical team at his disposal.
"But he was pleased to participate in a plan as a show of support for these marketplaces, which are providing quality, affordable health care options to more than a million people," the official said.
The Affordable Care Act, Obama's top domestic achievement to date, is designed to offer insurance to millions of Americans who have never been able to secure it before - some because of pre-existing health conditions like heart disease that insurers were unwilling to cover.
But a litany of problems with the government website, healthcare.gov, has played into the hands of Republicans, who say the federal government has no business intervening in the private health care market and should not be dictating health choices to Americans.
Obama told reporters on Friday at a year-end press conference that more than a million American had chosen new insurance plans using the new health care system.
The announcement about Obama's symbolic sign-up came on the day a deadline was meant to pass for people signing up for plans beginning January 1.
But the government extended the deadline by 24 hours, until late Tuesday, to avoid a crush of users on the website and possible problems, an administration official told AFP.
"We have programmed our systems to support January 1 coverage for those who attempt to complete their enrolment through the end of the day tomorrow," the official said on condition of anonymity.
A spokeswoman for the Centers for Medicare and Medicaid Services, Julie Bataille, said: "We recognize that many have chosen to make their final decisions on today's deadline and we are committed to making sure they can do so."
But despite the extension, Bataille urged people to "try to sign up today", saying: "You should not wait until tomorrow."