Minnetonka-based UnitedHealthcare on Thursday announced a plan to significantly increase its number of Medicare customers.

Starting in 2021, the insurer will sell Medicare Advantage plans in 291 new counties across the U.S., bringing the total number of counties covered to more than 2,100. In a news release, the company said the increase will mark the largest footprint expansion in five years.

Today, UnitedHealthcare serves more than 6.5 million people under the Medicare Advantage plan. With the expansion announced Thursday, that number could grow by more than 3 million people, company officials said.

Like regular Medicare plans, Medicare Advantage plans provide government-funded insurance for patients age 60 or older. However, unlike typical Medicare plans, Medicare Advantage plans are provided through a private-sector insurer; the government then pays the insurer to cover benefits. Nearly every major insurer in Minnesota offers Medicare Advantage plans, and they’ll all likely increase their coverage areas in the new year. (Medica and Allina Health Aetna both told the Star Tribune they plan to increase the number of counties covered in 2021.)

Still, UnitedHealthcare has captured the lion’s share of Medicare Advantage plans nationwide: As of March, the insurer serviced about a quarter of all Medicare Advantage plans in the U.S., according to Mark Farrah Associates, a Pennsylvania-based health care business research firm. That put UnitedHealthcare in first for percentage of market share, followed by Humana (18 percent) and CVS (10 percent).