A Roth conversion is reportable on the tax return of the year in which the Roth conversion was performed. For a Roth conversion performed in 2020, you'll get the Form 1099-R near the end of January 2021 and you'll report it on your 2020 tax return. This Roth conversion does not get reported on your 2019 tax return; it is not permitted to be treated as 2019 income.
