It comes after the Victorian government in Australia cracked down on so-called gay conversion therapy last year.
Washington will become the tenth US state to outlaw so-called "gay conversion” therapy on minors, in a move praised by rights groups who have called the practice "abusive".
A new bill stipulates that any licensed healthcare practitioner caught performing conversion therapy on a patient under the age of 18 could face fines or the revocation of their medical license.
Conversion therapy is already banned in the US states of California, Connecticut, Nevada, New Jersey, Oregon, Illinois, Vermont, New Mexico and Rhode Island.
The practice is also illegal in Switzerland, Brazil, Ecuador, Malta and Taiwan.
In Australia, the state of Victoria launched a watchdog last February to look into a range of health services complaints, including gay conversion.
Rights groups have commended the bill’s passing in Washington state.
“Children across the Evergreen State deserve to live their lives authentically and should never be subjected to the abusive practice of so-called conversion therapy,” said Human Rights Campaign Senior Vice President for Policy and Political Affairs JoDee Winterhof.
“It’s time for Washington to join the growing number of states and municipalities who are enacting these critical protections. We thank the state legislators who voted to protect LGBTQ youth from this dangerous practice and now call on Governor Inslee to sign this crucially important legislation.”
What is conversion therapy?
According to The Williams Institute at the University of California, Los Angeles, conversion therapy is a practice that aims to “cure” homosexuality.
It is practiced by some licenced medical professionals, and also by some clergy or spiritual advisors, in the name of religion.
The Williams Institute estimates 20,000 LGBTQIA minors in US states without protections will be subjected to conversion therapy should more states not issue a ban.
Talk therapy is the most common form of therapy used, though some practitioners have also used hypnosis and aversion techniques such as inducing nausea and vomiting, according to the Institute.
The Royal Australian and New Zealand College of Psychiatrists (RANZCP) also condemns conversion therapy.
The RANZCP Code of Ethics states that psychiatrists “shall not discriminate against patients on grounds of age, gender, race, ethnicity, sexual orientation, disability, language, religious or political affiliation”.
Conversion therapy remains legal in Australia.
During last year’s marriage quality postal survey the Australian Christian Lobby’s Lyle Shelton said parents should have the option to send their children to conversion therapy.
Mr Shelton told Buzzfeed while he did not think adults should be forced into conversion therapy, they should be able to decide whether their children should undergo it.
"I think anyone who wants to seek help for any issue they might be facing in their life should be free to do that," Mr Shelton said.
"Should people be forced to go to conversion therapy? No, absolutely not. Now, children — they are under the care and responsibility of their parents, so I think if someone's a minor, it is up to their parents. And I think parental rights should be respected.”
Victorian crackdown on conversion therapy
Washington’s new bill comes after Victoria launched a watchdog last February to look into a range of health services complaints, including gay conversion therapy.
The watchdog is led by lawyer Karen Cusack, who is Victoria’s first ever Health Complaints Commissioner.
"If they are found to be making false claims and to be acting in a manner that puts people's physical, mental or psychological health, safety or welfare at risk, the Commissioner will be able to ban them from providing such services," Victoria's health minister, Jill Hennessy, said in a statement in 2016.
“We have zero tolerance for any person purporting to be able to ‘convert’ gay people through medical or therapeutic means.”
Victorians can report a health practitioner if they feel a service they were given was unsatisfactory or lacked information or choice.
Patients can also report a provider if they feel they were denied respect, dignity or privacy.