If you repay a loan that was never treat as a deemed distribution or an offset distribution and, therefore, the outstanding loan amount never became a taxable distribution, you are paying back with the loan money that you were never taxed on, pre-tax money.
If you repay the loan after defaulting and the loan becoming a deemed distribution on which you must pay tax, your loan repayments become after-tax basis in the plan. When you eventually receive distributions from the plan, each distribution will be a prorated amount of taxable income and nontaxable basis.
Usually, though, when you have an outstanding loan and you leave the company and are therefore permitted to take regular distributions from the plan, the plan satisfies the loan by making an offset distribution, reducing the balance in the plan to your credit to satisfy the loan. An offset distribution is taxable unless you roll the roll an amount equal to the offset distribution into another qualified retirement account. For offset distributions that occurred prior to 2018, the rollover was required to be made within 60 days of the offset distribution, so it's too late to do such a rollover for an offset distribution that occurred before 2018. For offset distributions that occur in 2018 or later, the deadline for doing a rollover of the offset distribution is the due date of your tax return for the year in which the offset distribution occurred, including extensions. Of course you would need to come up with the money to complete the rollover because an offset distribution does not result in any money being paid to you since you already received the money as a loan.
