"We're confident that millions of Americans will choose to enroll when they learn that quality, affordable health insurance is within reach," said Health and Human Services Department Secretary Sylvia Burwell.
"We believe 13.8 million sign-ups during the upcoming open enrollment will help keep driving down the national uninsured rate, which is already the lowest in our nation's history," Burwell said.
Her projections come less than two weeks before the start of open enrollment, which will run from Nov. 1 through Jan. 31.
HHS also said that over the course of 2017, it estimates that an average of 11.4 million people will have so-called effectuated enrollment. For enrollment to be official, or effectuated, a person must pay their premium for the first month of coverage from their health plan.
For the first half of 2016, effectuated enrollment was 10.5 million people, officials said Wednesday.
It is normal in the insurance industry to see a drop between the total number of people who sign up for a plan, and the number of people who are enrolled. Some customers fail to pay their first premium, and others drop their plan during the year because they get coverage elsewhere, or for other reasons.
The estimates released Wednesday are for people who will be covered under plans sold on HealthCare.gov, the Obamacare exchange that serves 38 states, and the marketplaces run by individual states. It does not include people covered by individual plans — as distinct from insurance through an employer — sold outside of the government-run exchanges.
The estimates come as many states have approved sharply higher premium rates for 2017 plans sold on Obamacare exchanges, and several large insurers have significantly decreased the number of states where they sell such plans.
Most people who qualify to buy coverage on Obamacare exchanges also have low or moderate incomes that make them eligible for tax credits, or subsidies, that can significantly lower their premiums.
Those subsides, which almost 85 percent of enrollees get, can largely, or completely insulate them from the effects of premium rate increases. About 56 percent of Obamacare customers also have incomes low enough to qualify them for financial aid that reduces their out-of-pocket health costs.