I transferred funds from an IRA into the TSP. I have a basis reported on Form 8606 from the IRA. How do I offset my taxable TSP withdrawals for the basis from the IRA?

[dmertz · Accepted solution]

You can't.  Rollover of IRA basis to the TSP is, by law, not permitted.  The rollover to the TSP comes only from the pre-tax money in your account.

Did you roll over all money from your IRAs to the TSP or did you leave enough in your IRAs to cover your basis in nondeductible traditional IRA contributions?

If you rolled basis into the TSP, you'll either need to treat the basis as lost or you'll need to obtain a corrective distribution from the TSP of an amount equal to the basis that the TSP is not permitted to accept.