Utah on Tuesday became one of the most conservative states in the country to ban licensed therapists from performing conversion therapy on minors, curtailing a discredited practice that aims to change a person’s sexual orientation or gender expression.

The ban, which Gov. Gary R. Herbert, a Republican, proposed in November, added conversion therapy to a list of practices considered to be “unprofessional conduct” for state-licensed mental health therapists. Punishments could include suspending or revoking their license, according to state law.

The new rule does not apply to clergy members or religious counselors acting in a “religious capacity.” The rule also does not apply to parents or grandparents “acting substantially in the capacity of a parent or grandparent and not in the capacity of a mental health therapist.”

Still, L.G.B.T.Q. activists hailed it as a historic move.

“People have a lot of stereotypes about Utah, but over the past five years we've really been able to break through a very conservative culture to actually advance L.G.B.T. rights,” said Troy Williams, executive director at Equality Utah, a civil rights organization based in Salt Lake City.