I got married last year and my wife had a balance due on her 2017 tax return that she paid in 2018. How do I get a deduction for this?

[xmasbaby0]

The money you paid to the IRS for a previous year’s federal tax return is not deductible, so you do not enter it anywhere on your 2018 return.  If you paid taxes due in 2018 to your state or local government, you can enter that by going to Federal> Deductions and Credits> Estimates and Other Taxes Paid> Other Income Taxes.  You can enter the taxes you paid, but do not include any penalties or interest.

[SweetieJean]

Some states let you deduct Federal taxes paid on the state return.

[xmasbaby0]

Here they are: There are six states that allow you to enter the federal tax you paid on your state return--Alabama, Iowa. Louisiana, Missouri, Montana, and Oregon.  If you are filing in one of these states watch for the deduction when you prepare your state return.