The answer
Is Medicare Part D mandatory?
Medicare Part D is not mandatory, but going without it can cost you later.
Part D is the prescription drug part of Medicare, offered through private plans. You are not required to enroll, but if you do not sign up when you are first eligible and you do not have other qualifying drug coverage, you may owe a late enrollment penalty. That penalty is an amount added to your Part D premium, and it can last for as long as you have Part D coverage. In 2026, the national base premium that the penalty is calculated from is $38.99 per month. So Part D is optional, but skipping it without other creditable coverage can lead to a higher, lasting premium if you enroll later.