Effective Jan. 1, mental health practitioners are prohibited from performing sexual orientation change efforts -- known as reparative or conversion therapy -- for anyone under 18.
The therapies "have no basis in science or medicine and they will now be relegated to the dustbin of quackery," Brown said in a statement.
Mainstream associations representing psychiatrists and psychologists have dismissed reparative therapy in recent decades. A number of mental health associations in California -- including the state's Board of Behavorial Sciences, the California Association of Marriage and Family Therapists and the California Psychological Association -- supported the legislation.
But some organizations and ministries continue to use counseling and prayer to try to help conflicted Christians rid themselves of unwanted homosexual inclinations. Gay rights activists have said the damage they inflict on individuals can be deep and lasting and can put youth at higher risk of depression and suicide.
"We're grateful to Gov. Brown for standing with California's children," the Human Rights Campaign said in a statement. "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."
The group called on other states to follow California's lead on the issue.
Conservative religious groups and some Republicans have argued that banning conversion therapy would hinder parents' right to provide psychological care for children experiencing gender confusion.
The Encino, Calif.-based National Association for Research and Therapy on Homosexuality said in August that the bill was a case of "legislative overreach." They said Democratic state Senator Ted Lieu's claims of harm to children were based on politics, not research.
Lieu, who sponsored the bill, said the law will stop children from being psychologically abused.
Brown signed the bill Saturday amid a busy weekend that saw him act on more than 100 pieces of legislation, including a bill he signed late Sunday that would allow some illegal immigrants to get California drivers licenses.
The bill lets the Department of Motor Vehicles issue licenses to illegal immigrants eligible for work permits under a new Obama administration policy. It requires the department to accept as proof of legal residence whatever document the federal government provides to participants in its deferred action program.
The governor also signed into law a bill that will allow parents who don't have their children vaccinated to enroll their children in school. Parents would have to obtain a waiver from a physician or a nurse practitioner saying they received information about the benefits and risks of immunization.
Brown vetoed a bill that could have protected illegal immigrants from deportation if they committed minor infractions. The bill was dubbed "anti-Arizona" legislation, a reference to that state's immigrant identification law.
He also vetoed legislation that would have provided overtime pay, meal breaks and other labor protections to an estimated 200,000 caregivers, nannies and house cleaners. The Democratic governor said they deserve fair pay and safe working conditions but said the bill "raises a number of unanswered questions."