Humana says it will cover the 36 Mississippi counties that had been left uncovered on the new federal online marketplace when it starts enrolling customers in October.
The move means consumers in all counties will be able to buy health insurance through the online exchange. Coverage under those policies begins Jan. 1.
Until Friday, insurers had announced plans to only cover 46 of the state's 82 counties.
Of states where insurers had been announced for exchanges, Mississippi had been the only one with areas that no insurer had filed to serve.
Humana, based in Louisville, Ky., had said it would cover people in four urban Mississippi counties, Hinds, Madison, Rankin and DeSoto.
Magnolia Health Plan, a unit of St. Louis-based Centene Corp., has said it will serve 46 counties.