Psychotherapy that seeks to change minors' sexual orientation or gender identity is now illegal in Colorado.
Gov. Jared Polis signed the ban into law on Friday during a Capitol ceremony.
Psychology's governing bodies have denounced so-called conversion therapy, which treats being gay or transgender as a mental illness.
Research shows it increases the risk of suicide, drug abuse and depression among teens.
Polis, who is gay, says he's thrilled to launch LGBTQ Pride Month in June by outlawing what he calls a "tortuous practice" harming children.
Colorado becomes the 18th U.S. state — and the fourth this year — to ban conversion therapy.
Polis also signed into law a bill making it much easier for transgender Coloradans to update the gender on their birth certificates, identification documents and driver's licenses.