Updated July 28, 2026. Quick answer: Yes. Montana taxes IRA and plan distributions in full with no general retirement exclusion, so a Roth conversion is taxed as ordinary income at the state rate on top of your federal bill.
Why a conversion is a different question from a withdrawal
Almost every state summary answers “how does Montana tax retirement income?” That is a question about distributions. A Roth conversion is not a distribution in the ordinary sense — it is a voluntary election to recognise income now in exchange for tax-free growth later. Whether a state’s retirement exclusion reaches that election is a separate question, and it is the one that decides your bill.
There is no exclusion to argue about here, which at least makes the maths clean: the converted amount is added to Montana taxable income in the conversion year.
Because the state cost is certain, the lever that matters in Montana is when and how much — splitting a conversion across tax years, or converting in a year of unusually low income, changes the bill in a way that arguing about the exclusion cannot.
What Montana does with the converted amount
State income tax: graduated, 2 brackets: 4.7% / 5.65% for 2026 (top rate cut from 5.9% by HB 337; falls to 5.4% in 2027; top bracket starts ~$47,500 single / ~$95,000 joint)
How Montana treats IRA and plan income: Pensions, 401(k), and IRA distributions fully taxable — the old partial pension/annuity exemption was repealed with the TY2024 restructure. Taxpayers 65+ receive a $5,500 subtraction (any income type) plus the state follows the larger federal standard deduction.
| Conversion | State tax at 4.7% |
|---|---|
| $50,000 | $2,350 |
| $100,000 | $4,700 |
| $250,000 | $11,750 |
Straight arithmetic at the stated rate. It ignores deductions, credits, and any graduated bracket effects, so treat it as the ceiling rather than a filing figure.
What to ask a preparer about Montana
None on eligibility — it is taxable. Ask instead how splitting the conversion across two tax years changes the bracket outcome.
Converting around a move
Converting in the year you move into Montana is the case that catches people. Residency at the moment of conversion is what generally determines which state gets to tax it, so a conversion executed a week before a move and one executed a week after can produce different bills.
Four separate taxes change when you move, not one: income tax on withdrawals, treatment of Social Security, estate tax, and inheritance tax. A state that looks good on conversions can be worse on the other three.
The state bill is the smaller half
Whatever Montana does, the conversion is federal ordinary income first. The federal bracket you land in, and whether the conversion pushes you over an IRMAA threshold two years later, will usually move more money than the state line does. The state answer tells you whether to convert here; the federal answer tells you how much to convert at once.
Paying the tax from outside the account matters more than either. Using converted dollars to pay the bill shrinks the balance that was the entire point of converting.
Sources
Authority: Mont. Code Ann. § 15-30-2120 (subtractions incl. 65+ and military); SB 399 (2021); HB 337 (2025); MT DOR: Working Military Retirement Exemption.
Compiled from state statutes, session laws and revenue-department publications and adversarially verified in July 2026. Dataset confidence for Montana: high.
This page states what the cited authority says. It is not tax advice, and a conversion large enough to matter is worth putting in front of a preparer who can see your whole return.