Here is how Medicare enrollment impacts your ability to contribute or use your health savings account.
HSA contributions and Medicare enrollment: Not permitted
Once you enroll in Medicare, your HSA contribution limit becomes zero, starting the first month of enrollment. This applies even to retroactive Medicare coverage — if your enrollment is backdated, any contributions you made during that backdated period are considered excess contributions.
Source: IRS Publication 969, Health Savings Accounts and Other Tax-Favored Health Plans
You must stop HSA contributions by the first day of the month you enroll in Medicare, to avoid taxes and penalties.
Retroactive Part A coverage
If you qualify for premium-free Medicare Part A, your coverage can be applied retroactively up to 6 months from your enrollment date. Plan to stop HSA contributions 6 months before enrolling in Medicare Part A and Part B (or before applying for Social Security retirement benefits, if you're applying before you stop working).
Source: CMS Fact Sheet, "Deciding whether to enroll in Medicare Part A and Part B when you turn 65
HSA distributions and Medicare enrollment: Permitted
While you can't contribute to an HSA while enrolled in Medicare, you can continue to make distributions (withdrawals) from an already-established HSA for qualified medical expenses. HSA funds are truly your funds!
Source: IRS Notice 2004-2, Q&A 27