This is a great question, and here’s the answer straight from the horse’s mouth:
Yes. When you become eligible for Medicare, you’ll need to enroll in both Part A and Part B to get full benefits from your retiree plan. Your retiree plan usually offers benefits that fill in Medicare’s gaps in coverage and sometimes includes extra benefits, like prescription drugs. Remember, retiree coverage isn’t a Medigap policy. Since retiree coverage isn’t based on current employment, if you delay enrolling in Part B when you’re first eligible, you won’t be eligible for a Special Enrollment Period to enroll in Part B later.
If you’re not sure how your plan works with Medicare, get a copy of your plan’s benefits booklet. For more information how your retiree coverage works, call your benefits administrator.
A related question: I’m a retired Federal employee with Federal Employee Health Benefits Plan (FEHBP) and am eligible for Medicare. Do I need to enroll in Part B?
Enrolling in Part B is your choice. Federal retirees are offered the same benefits as all other retirees. You can enroll in Part B during your Initial Enrollment Period. If you don’t enroll in Part B when you’re first eligible and decide to enroll later, you may have to pay more for as long as you have it. Get more information about how FEHBP works with Medicare from the U.S. Office of Personnel Management.