A TSP loan that you are still paying back and on which you have not defaulted is not taxable.
A TSP loan only becomes taxable if you default on the loan and receive a Form 1099-R with code L in box 7 or the loan is satisfied with an offset distribution that reduces the balance to you credit in the TSP and the TSP issues a Form 1099-R that includes code M in box 7, usually upon separation from service while the loan is still outstanding. If you received either of these Forms 1099-R, they must be reported on your tax return.
