The date has moved back a week, from Dec. 15 to Dec. 23.
The delay was announced after the White House agreed with insurers, who were concerned that technical issues with HealthCare.gov were preventing many users from applying for coverage.
A spokesman for the Centers for Medicare and Medicaid Services told reporters that the deadline extension allows enough time for carriers to process enrollment applications by the new year.
Meanwhile, the administration is working feverishly to ensure HealthCare.gov will handle a late-December traffic surge as a result of the announcement.
The White House hopes the change will reassure carriers who are worried over instability in the insurance market and resulting rate increases in coming years.