Do I have to enroll in Medicare at 65 if I have an HSA?

The answer changes with the size of your employer. If the employer has 20 or more employees, its group plan pays before Medicare and counts as creditable coverage, so delaying Medicare is generally safe. If the employer has fewer than 20 employees, Medicare is supposed to pay first at 65, and skipping it can leave you effectively uninsured for the portion Medicare would have paid. In that case, delaying to protect HSA contributions may cost far more than the tax break is worth.

The rules that interact

1. Enrolling in any part of Medicare, including premium-free Part A, ends your eligibility to contribute to an HSA under IRS rules. You keep the account and can spend it tax-free, you just cannot add to it.
2. Claiming Social Security automatically enrolls you in Part A. You cannot take Social Security retirement benefits and keep contributing to an HSA.
3. If you enroll in Part A after 65, coverage is backdated up to 6 months (but not earlier than your 65th birthday month). Contributions made during that backdated window become excess contributions.
4. Delaying Part B is penalty-free only while you have coverage from current employment. For 2026, the Part B late penalty is 10% of the $202.90 standard premium for each full 12-month gap, permanently. When employment ends, an 8-month Special Enrollment Period lets you enroll without penalty.

The 6-month backdating trap, worked through

Example: you plan to retire in June 2027 at 66 and apply for Medicare that month. Part A will be backdated 6 months, to December 2026. To avoid excess contributions, you would stop HSA contributions after November 2026, a full 6 months before you apply, and prorate your final-year contribution limit by the months you were still HSA-eligible. Contributing past that point can trigger a 6% excise tax each year the excess stays in the account, depending on when you catch it.

What this means in practice

If you work past 65 at a larger employer with an HSA-qualified high-deductible plan, you may delay Medicare, keep contributing, and enroll later with no penalty. Just calendar two dates: the day you plan to file for Medicare or Social Security, and the date 6 months before it, which is your last month of HSA contributions.

Here’s the mistake I see people make: they enroll in “free” Part A at 65 just because it costs nothing, then keep funding the HSA at work. Nothing flags the error at the time. The IRS math catches up at tax time.