Two licensed therapists and one aspiring therapist filed a suit against the law, which is due to go into effect on Jan. 1.
California's Democratic Gov. Jerry Brown in September signed into law a ban against the 'conversion therapy' for children and teenagers, making the nation's most populous state the first in the country to do so.
The law bars therapists from performing sexual-orientation change counseling with children and teenagers under 18 and was supported by the California Psychological Association.
Gay rights advocates say the therapy can psychologically harm gay and lesbian youths, leading to depression or even suicide. They say the treatment, also called reparative therapy, has no medical basis because homosexuality is not a disorder.
Lawyers for the plaintiffs in the case, filed by the Christian legal group Pacific Justice Institute, argued that the law violates constitutionally protected rights to free speech and freedom of religion.
Granting the plaintiffs a preliminary injunction, Judge Shubb ruled that their claims were likely to succeed based on freedom of speech violations.
They had also shown they were likely to suffer "irreparable harm" in the absence of an injunction, he said in written ruling just hours after a hearing on the matter.
Attorneys for the state were joined by lawyers from Equality California, which was a sponsor of the bill, in arguing that there is substantial evidence that the conversion therapy practice causes harm to those who undergo it.
The National Center for Lesbian Rights said it was disappointed with the decision, but stressed the temporary nature of the ruling and its limited scope.
"We are confident that as the case progresses, it will be clear to the court that this law is fundamentally no different than many other laws that regulate health care professionals to protect patients," the organization's legal director, Shannon Minter, said in a statement.
Shubb expressed concerns during the earlier hearing that banning licensed practitioners from offering the therapy would only drive parents to seek out the treatment from unlicensed providers.
"That's what they used to do with abortions. It didn't work very well," he said, mentioning back-alley procedures that endangered women's health. "It may also be the opposite of what the state was trying to accomplish."
An attorney for Equality California, Michelle Friedland, argued it would be unethical for therapists to refer their clients to a treatment that was scientifically discredited.
Another similar suit seeking a separate injunction against the law was argued on Friday, and the judge in that case indicated she would rule this week.