A TSP loan feels tax-free when you take it, and it is. The loan principal is your own money, coming out of your TSP account and coming back to you. There is no tax on the receipt and no tax on the repayment, provided everything goes right.

The tax story changes the moment one of three things goes wrong: you separate from federal service, you default on the loan, or you fail to repay on schedule. In each of those cases, what was a tax-free flow of your own money becomes a taxable distribution and, potentially, a distribution subject to the 10% early withdrawal penalty.

The mechanics of a TSP loan.

A TSP loan is a participant loan: you borrow from your own TSP account, you pay yourself back with interest, and the principal and interest both flow back into your TSP balance. There is no credit check, no underwriting, and no third-party lender.

The TSP supports two loan types:

  • General purpose loan. Repaid over 1 to 5 years. Used for any purpose. No documentation required.
  • Residential loan. Repaid over 1 to 15 years. Used for the purchase or construction of a primary residence. Documentation of the home purchase is required.

The minimum loan amount is $1,000; the maximum is the lesser of $50,000 (minus your highest outstanding loan balance in the past 12 months) or half your vested TSP balance. The interest rate is the G Fund rate at the time the loan is initiated, plus 1%. Interest is paid into your own TSP account.

None of this is taxable on receipt. None of it is taxable on repayment, as long as everything goes right.

What changes when you separate from service.

When you separate from federal service — whether through retirement, resignation, or other reasons — the TSP gives you a window to repay any outstanding loan balance. The window:

  • For a general purpose loan, the repayment window is typically up to 90 days from separation, though specific TSP rules apply and the window can vary by circumstance.
  • For a residential loan, the TSP allows the loan to continue to be repaid on its original amortization schedule, even after separation, provided the installments are paid by direct debit or another approved mechanism.

If you repay the loan in full within the window, there is no tax consequence. The loan closes cleanly.

If you don’t repay the loan in full within the window, the unpaid balance is declared as a taxable distribution. The TSP issues a Form 1099-R for the unpaid balance, with federal income tax withheld at 20% mandatory withholding, and the distribution is reported as ordinary income.

If you are under age 59½ at the time of the deemed distribution, an additional 10% early withdrawal penalty applies under IRC § 72(t), on top of the ordinary income tax.

What changes on default.

A TSP loan can also default during active employment. The most common ways:

  • Missed installment payment. If your payroll deduction fails or you miss a payment, the loan is at risk of being declared in default.
  • Failure to certify employment status. The TSP periodically requires loan recipients to certify that they are still employed. Failure to certify can trigger default treatment.

A defaulted loan is treated similarly to a separated borrower who failed to repay: the unpaid balance is declared as a taxable distribution in the year of default, with the same 20% mandatory withholding and the same potential 10% early withdrawal penalty.

The TSP’s default mechanism is not a friendly process. There is no cure period of meaningful length, and the tax consequences accrue from the date of the declared default.

The pro-rata wrinkle.

One of the most counter-intuitive tax issues with TSP loans involves the pro-rata rule. When a TSP loan is taken, the loan amount is treated as coming pro-rata from the participant’s Traditional and Roth balances. The same pro-rata treatment applies to repayments.

The wrinkle appears when a portion of the loan is later declared as a taxable distribution. If, for example, the deemed distribution occurs after the participant has shifted their contribution mix heavily toward Roth, the pro-rata calculation can produce a deemed distribution that includes a Roth component — which, while still taxable in the sense of being reportable, has different tax treatment than a Traditional deemed distribution.

The pro-rata rule can also create an unwanted consequence: if you have been making Roth contributions only, and you take a loan that is later deemed distributed, the deemed distribution includes a Roth portion (and a Traditional portion). The Roth portion is taxable in the same way as a Roth conversion that fails the 5-year rule.

The pro-rata rule is mechanical, and it is rarely what borrowers expected. It deserves attention before the loan is taken, not after the deemed distribution arrives.

What the tax bill actually looks like.

For a separated or defaulted borrower under 59½ with a Traditional TSP balance, the tax consequences of an unpaid TSP loan typically include:

  • Federal income tax at the marginal rate, on the unpaid balance.
  • 20% mandatory federal withholding at the time the TSP declares the distribution.
  • 10% early withdrawal penaltyunder IRC § 72(t), if under 59½ with no exception.
  • State income tax in most states, on the unpaid balance, at the state marginal rate.
  • Possible excise tax if the unpaid balance is large enough to push the participant into a higher bracket.

The combined effective rate, including the 10% penalty, can reach 35% to 45% of the unpaid balance. That is not the same thing as the loan being “your own money anyway.” The tax code does not see it that way.

The honest comparison to a withdrawal.

A TSP loan, taken and repaid on schedule, is tax-free and cheaper than a withdrawal. A TSP loan that goes bad is often more expensive than a withdrawal would have been, because the deemed distribution carries both the income tax and the early-withdrawal penalty that a planned withdrawal would have.

The decision to take a TSP loan should weigh both scenarios. The cleanest borrowers — who take a short loan, repay on schedule, and separate only after the loan is closed — get the cheapest experience. The messiest borrowers — who take a long loan, separate mid-loan, and don’t repay within the window — get the most expensive experience the TSP system offers.