Colorado voters approved Proposition 135 in the November 3, 2026, midterm elections, prohibiting healthcare professionals from performing certain gender-affirming surgeries on individuals under 18. The Associated Press called the measure on November 6, with approximately 52% of voters supporting the ban.

The measure prohibits surgical procedures intended to alter a minor's biological sex characteristics in response to the individual's perception of their sex or gender. It also bars government funding and insurance coverage for the prohibited procedures, extending the restrictions to both public and private insurance plans.

Proposition 135 does not prohibit all forms of gender-related medical care for minors. Hormone therapy, puberty-blocking medications, counseling, and other nonsurgical treatments are not restricted under the measure. It also includes exceptions for circumcision and treatment of medically verifiable disorders of sex development and certain acquired physical or chemical abnormalities.

Supporters argued that irreversible surgical procedures should be restricted until individuals reach adulthood. Opponents contended that medical decisions should remain between patients, parents, and healthcare providers, rather than being determined by state law. Critics also raised concerns that the measure's language could affect certain procedures involving minors who are not transgender.

The measure is a change to state statute rather than the Colorado Constitution, meaning the state Legislature can amend or repeal it through the legislative process. Democratic Governor-elect Phil Weiser has indicated that the measure, along with a separate voter-approved restriction on transgender students participating in school sports, will receive further scrutiny before implementation.