Mont. Code Ann. § 40-1-402

40-1-402 . Declaration of invalidity. (1) The district court shall enter its decree declaring the invalidity of a marriage entered into under the following circumstances:

(a) a party lacked capacity to consent to the marriage at the time that the marriage was entered into, either because of mental incapacity or infirmity or because of the influence of alcohol, drugs, or other incapacitating substances, or a party was induced to enter into a marriage by force or duress or by fraud involving the essentials of marriage;

(b) a party lacks the physical capacity to consummate the marriage by sexual intercourse, and at the time that the marriage was entered into, the other party did not know of the incapacity;

(c) a party was under 16 years of age or was 16 or 17 years of age and did not have the consent of the party's parents or guardian or judicial approval; or

(d) the marriage is prohibited.

(2) A declaration of invalidity under subsections (1)(a) through (1)(c) may be sought by any of the following persons and must be commenced within the times specified, but a declaration of invalidity may not be sought after the death of either party to the marriage:

(a) for lack of capacity to consent because of mental incapacity or infirmity, no later than 1 year after the petitioner obtained knowledge of the described condition;

(b) for lack of capacity to consent because of the influence of alcohol, drugs, or other incapacitating substances, no later than 1 year after the petitioner obtained knowledge of the described condition;

(c) for lack of capacity to consent because of force, duress, or fraud, no later than 2 years after the petitioner obtained knowledge of the described condition;

(d) for the reason set forth in subsection (1)(b), by either party, no later than 4 years after the petitioner obtained knowledge of the described condition;

(e) for the reason set forth in subsection (1)(c), by the underaged party or the party's parent or guardian, before the time that the underaged party reaches the age at which the party could have married without satisfying the omitted requirement.

(3) A declaration of invalidity for the reason set forth in subsection (1)(d) may be sought by either party, the legal spouse in case of a bigamous marriage, the county attorney, or a child of either party, at any time before the death of one of the parties.

(4) Children born of a marriage declared invalid are legitimate.

(5) Unless the court finds, after a consideration of all relevant circumstances, including the effect of a retroactive decree on third parties, that the interests of justice would be served by making the decree not retroactive, it shall declare the marriage invalid as of the date of the marriage. The provisions of chapter 4 relating to property rights of the spouses, maintenance, support, and parenting of children on dissolution of marriage are applicable to nonretroactive decrees of invalidity.

(6) The clerk of the court shall give notice of the entry of a decree declaring the invalidity of a marriage:

(a) if the marriage is registered in this state, to the clerk of the district court of the county where the marriage is registered, who shall enter the fact of invalidity in the book in which the marriage license and certificate are recorded; or

(b) if the marriage is registered in another jurisdiction, to the appropriate official of that jurisdiction, with the request that the official enter the fact of invalidity in the appropriate record.