Not immediately, if you have good employer insurance and you do not need Medicare to supplement your coverage. There is a period of time when your employer or retiree health plan will pay first and Medicare will pay second. This is called a 30-month coordination period and it starts when you first qualify for Medicare coverage, even if you have not signed up for it yet.
At the end of the 30-month coordination period, Medicare will pay first for all Medicare-covered services and your employer group health plan or retiree plan will pay second. (Your employer group or retiree health plan may also pay for services not covered by Medicare. Call your benefits administrator to find out.)
If you are happy with your employer coverage, you can delay enrollment in Medicare until the end of the 30-month coordination period. This way, you will not have to pay monthly Medicare Part B premiums while you do not need Medicare coverage. To avoid incurring a premium penalty when you enroll in Medicare later, do not enroll in Medicare Part A or Part B when you first become eligible for Medicare.
To prevent a gap in coverage, enroll in Medicare Part A and B a few months before your 30-month coordination period ends. Tell Social Security that you want your Medicare coverage to begin the first month after the coordination period ends, when Medicare will become your primary coverage. Apply early to give Social Security time to process your application.