Bill defining sex as strictly biological throughout Montana signed into law

HELENA, Mont. — Senate Bill 437, establishing a strictly biological definition of sex throughout Montana, is now state law.

Governor Greg Gianforte today signed the bill which defines sex as determined exclusively by reproductive anatomy and the type of gamete a person is capable of producing.

According to the legislation, there are only two sexes: male and female, based on sperm or egg production.

The legislation amends existing Montana statutes across employment, education, healthcare and criminal justice to replace previous definitions with this biological-based framework.

The law affects numerous areas of state operations. School districts would have to notify parents before a child uses a pronoun that does not align with the child's biological sex as defined in the law. If parents provide written consent for such pronoun use, no person may be compelled to use pronouns that do not align with the child's biological sex.

Athletic teams sponsored by public schools or institutions would have to be designated as males, men or boys; females, women or girls; or coed or mixed. Teams designated for females, women or girls may not be open to students of the male sex.

The legislation also impacts marriage law, maintaining that marriages between persons of the same sex are prohibited.

In correctional facilities, the law would designate "inmate" as a male or female offender under sentence to or confined in a prison or correctional institution.

Reagor v State of Montana, a legal case filed against SB 458, was ruled unconstitutional in Missoula County District Court before going to the Montana Supreme Court in early 2026 on appeal where the case remains pending.

The high court is expected to review the bill in April.