In all, 9 million people have selected coverage, Sebelius said. That includes private marketplace coverage, Medicaid and the 3 million young adults who have already gained coverage because the Affordable Care Act allows them to stay on their parents' health plans until age 26.
The report released Monday covered enrollment through Dec. 28, 2013. It's the federal government's third report since the marketplaces opened on Oct. 1, 2013.
Enrollment surged during December, with 1.8 million people selecting a health plan -- a five-fold increase over the number of consumers who signed up in October and November combined.
Seventy-nine percent of people signing up for marketplace insurance plans have received financial assistance in the form of tax credits.
By gender, a greater percentage of women than men (54 percent versus 46 percent) have enrolled for "Obamacare" through the online marketplaces.
The federal registration portal -- HealthCare.gov -- received more than 53 million website visits through the Oct. 1-Dec. 28 period. State and federal call centers have received more than 11 million phone calls, the report said. HealthCare.gov was initially plagued by computer glitches that made the site almost unusable for the first several weeks of operation.
It's still not known how many people who signed up for coverage have paid their premiums -- the final step for ensuring coverage.
Julie Bataille, a spokeswoman for Centers for Medicare & Medicaid Services' Office of Communications, said the federal government is "ramping up outreach activities" to make sure people know they can enroll in marketplace coverage and pay those premiums.
The Congressional Budget Office had estimated last year that 7 million people would sign up for Obamacare through the online exchanges by 2014.
The federal exchange, HealthCare.gov, is the gateway for people to sign up in 34 states. Sixteen states and the District of Columbia are running their own exchanges.