I made a nondeductible traditional IRA contribution for 2025, but converted it to Roth in 2026. Which tax return reports what?

Split-year backdoor Roth reporting is a two-return process. If you made a 2025 nondeductible traditional IRA contribution but did the Roth conversion in 2026, your 2025 return and 2026 return each report different pieces of the transaction.

Your 2025 return reports the contribution:

• Report the nondeductible traditional IRA contribution on 2025 Form 8606, Part I
• This establishes your basis so the IRS knows that contribution was already taxed
• You generally do not report the Roth conversion on the 2025 return if the conversion actually happened in 2026

Your 2026 return reports the conversion:

• You will usually receive a 2026 Form 1099-R in early 2027
• Report the Roth conversion on 2026 Form 8606
• Only the pre-tax portion and any earnings are taxable. Your nondeductible basis is not taxed again

This is one of the most common points of confusion with backdoor Roths, especially when the contribution is made between January 1 and the filing deadline for the prior year. The contribution can be assigned to 2025, but the conversion is taxed based on when it actually occurred, which is 2026 in your example.

Practical tip: keep a copy of your 2025 Form 8606. Your 2026 tax software or preparer will need that basis information to prevent the conversion from being taxed twice.