Can I pay a medical bill now and reimburse myself from my HSA years later?

Yes, and it's one of the HSA's most powerful features. The IRS places no time limit on reimbursement, so you can pay a qualified medical expense out of pocket today, leave the HSA invested to grow tax-free for years or decades, then withdraw that amount tax-free later using the old receipt as justification. This turns the HSA into a stealth retirement account: contributions are deductible, growth is untaxed, and the eventual reimbursement is tax-free. The rules: the expense must have been incurred after you opened the HSA, it must be a qualified medical expense, and you must not have already deducted it or paid it from another HSA. Keep meticulous records and receipts, since you may reimburse decades later.