Montana Prenuptial Agreement Laws: Requirements and Enforcement

Montana has adopted the Uniform Premarital Agreement Act, codified at MCA 40-2-601 to 40-2-610. A premarital agreement under the act "must be in writing and signed by both parties" (MCA 40-2-604), takes effect on marriage, and is not enforced if the challenging spouse proves it was signed involuntarily or meets the act's unconscionability-and-disclosure test (MCA 40-2-608).

Information last verified on 2026-10-08. This article has not been reviewed by a licensed lawyer.

Jurisdiction scope: This article covers premarital agreements under Montana's Uniform Premarital Agreement Act, MCA 40-2-601 to 40-2-610, with a note on property division under MCA 40-4-202. It does not cover postnuptial agreements, Montana case law on premarital agreements, the general statute of frauds, which agreements the act applies to by date of signing, or other states' law. It is not a drafting guide.

Which law governs prenups in Montana?

Montana's premarital agreement statute is Part 6 of Title 40, chapter 2. MCA 40-2-601 gives the part its short title, the "Uniform Premarital Agreement Act." The part runs from 40-2-601 to 40-2-610.

The act governs formalities, when the agreement takes effect, the grounds for refusing enforcement, amendment and revocation, what happens if the marriage is declared invalid, and the time limits for enforcement claims. This page does not cover which agreements the act applies to based on when they were signed.

Requirements for a valid Montana prenup

MCA 40-2-604 sets the formalities in two sentences:

"A premarital agreement must be in writing and signed by both parties. It is enforceable without consideration."

Section 40-2-604 does not mention notarization or witnesses. This page does not cover whether other Montana statutes, such as the general statute of frauds, add requirements; a Montana lawyer can advise on how a specific agreement should be signed.

The agreement does not operate before the wedding. MCA 40-2-606 provides: "A premarital agreement becomes effective upon marriage."

When a Montana court will not enforce a prenup

MCA 40-2-608(1) places the burden on the spouse who wants to avoid the agreement. The agreement "is not enforceable if the party against whom enforcement is sought proves" one of two things.

1. Involuntary signing. "(a) that party did not execute the agreement voluntarily."

2. Unconscionability plus a disclosure failure. The second ground requires every one of these elements:

"(b) the agreement was unconscionable when it was executed and, before execution of the agreement, that party: (i) was not provided a fair and reasonable disclosure of the property or financial obligations of the other party; (ii) did not voluntarily and expressly waive, in writing, any right to disclosure of the property or financial obligations of the other party beyond the disclosure provided; and (iii) did not have or reasonably could not have had an adequate knowledge of the property or financial obligations of the other party."

The elements are joined by "and." Under the statute's text, an agreement that was unconscionable when signed is still enforced under this ground if the challenging spouse received fair disclosure, waived disclosure in writing, or had or could reasonably have had adequate knowledge of the other's finances.

Under MCA 40-2-608(3), "An issue of unconscionability of a premarital agreement must be decided by the court as a matter of law."

Independent lawyers and timing

Sections 40-2-604 through 40-2-608 do not contain a requirement that each party have a separate lawyer, or a waiting period between receiving the agreement and signing it. This page does not cover how Montana courts assess voluntariness under 40-2-608(1)(a). A lawyer licensed in Montana can draft or review an agreement.

Waiving alimony (maintenance) in a Montana prenup

The act limits how far a support waiver can go. Under MCA 40-2-608(2), if a provision modifying or eliminating spousal support would leave a spouse eligible for public assistance, "a court, notwithstanding the terms of the agreement, may require the other party to provide support to the extent necessary to avoid that eligibility." For how maintenance works without an agreement, see Montana alimony laws.

What a prenup cannot do in Montana

A premarital agreement cannot reduce what a child is owed. MCA 40-2-605(2) states: "The right of a child to support may not be adversely affected by a premarital agreement." Terms on other matters are allowed only if they do not violate public policy or a statute imposing a criminal penalty (MCA 40-2-605(1)(h)), and support waivers have the limits described above. This page does not cover how Montana treats agreement terms on parenting or custody.

Changing or cancelling a prenup after the wedding

MCA 40-2-607 provides: "After marriage, a premarital agreement may be amended or revoked only by a written agreement signed by both parties." The amended agreement or the revocation is enforceable without consideration.

If the marriage is declared invalid

If a court later declares the marriage invalid (Montana's form of annulment), MCA 40-2-609 limits the agreement: "If a marriage is declared invalid, an agreement that would otherwise have been a premarital agreement is enforceable only to the extent necessary to avoid an inequitable result."

Time limits for enforcing a prenup

MCA 40-2-610 pauses the clock during the marriage: "Any statute of limitations applicable to an action asserting a claim for relief under a premarital agreement is tolled during the marriage of the parties to the agreement." Equitable defenses such as laches and estoppel remain available to either party.

Postnuptial agreements

This page does not cover agreements signed after the wedding that are not amendments to a premarital agreement. Ask a Montana lawyer how an agreement signed during the marriage is treated.

How Montana divides property without a prenup

Montana is not a community-property state. On dissolution, the court, "without regard to marital misconduct," shall "finally equitably apportion between the parties the property and assets belonging to either or both, however and whenever acquired" (MCA 40-4-202(1)). A premarital agreement is how couples set different terms. See Montana divorce laws.

Disclaimer: This article provides general legal information about premarital agreements under Montana's Uniform Premarital Agreement Act, MCA 40-2-601 to 40-2-610. It is not legal advice and is not a template or drafting guide. The information was verified on 2026-10-08, and the law can change. For advice about a specific agreement, consult a lawyer licensed in Montana.

Related

• Prenuptial agreement laws in every state
• Montana divorce laws
• Montana alimony laws
• Montana child support laws

Last updated: 2026-10-08.

Frequently Asked Questions

Are prenups enforceable in Montana?

Yes, under the Uniform Premarital Agreement Act, MCA 40-2-601 to 40-2-610. A court will not enforce one if the challenging spouse proves involuntary signing or the unconscionability-and-disclosure test in MCA 40-2-608(1).

Does a Montana prenup have to be notarized?

MCA 40-2-604 requires a writing signed by both parties and does not mention notarization. A Montana lawyer can advise whether a particular agreement should be notarized or witnessed.

Can a prenup waive alimony in Montana?

It can modify or eliminate spousal support, but if that would leave a spouse eligible for public assistance, a court may require support to the extent needed to avoid that eligibility (MCA 40-2-608(2)).

Can a Montana prenup limit child support?

No. MCA 40-2-605(2) provides that the right of a child to support may not be adversely affected by a premarital agreement.

Can we change our prenup after we marry in Montana?

Yes, but only by a written agreement signed by both spouses (MCA 40-2-607).

Is a prenup invalid if my spouse hid assets in Montana?

Lack of fair disclosure counts only together with the other elements of MCA 40-2-608(1)(b): the agreement must also have been unconscionable when signed, and the challenging spouse must not have waived disclosure in writing or had adequate knowledge of the other's finances.