becomes first state to ban therapy that attempts to change sexual orientation.
Psychotherapy that attempts to turn gay teenagers straight has been banned in California.
Governor Jerry Brown signed legislation to change the law, which will come into effect on January 1.
California has become the first state to ban this controversial form of therapy, with Brown saying they have "no basis in science or medicine".
The move has been welcomed by many mental health associations, including the Human Rights Campaign, which aims to gain equality for LGBT people in the US.
Chad Griffin, president of the campaign, said: "We're grateful to Governor Brown for standing with California's children. LGBT youth will now be protected from a practice that has not only been debunked as junk science, but has been proven to have drastically negative effects on their well-being."
From January 1, mental health practitioners will be banned from using reparative or conversion therapy, which attempts to change sexual orientation, on any below the age of 18.
The bill was first introduced by Senator Ted Lieu in February this year. He said: "No one should stand idly by while children are being psychological abused, and anyone who forces a child to try to change their sexual orientation must understand this is unacceptable. Governor Brown should be commended for protecting LGBT youth by ending this type of quackery."
Opponents to the bill include The Encino, a California-based National Association for Research and Therapy on Homosexuality, which said the bill was "legislative overreach".
According to the LA Times, Republican lawmakers say it places restrictions on how parents want to raise their children. Matthew B McReynolds, a staff attorney for the Pacific Justice Institute told the news provider: "In our view, it's an intrusion beyond what the government has done before."
The Human Rights Campaign said it hopes all states will follow California in banning these forms of psychotherapy. Griffin added: "We will continue our fight against this kind of child abuse, which has been deemed harmful to children by all major mental health, medical, and child welfare organisations."
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