Waldemar de Carvalho, a federal judge in the capital Brasilia, backed a psychologist who had her licence revoked for offering so-called "conversion therapy".
Rozangela Justino is an evangelical Christian who has called homosexuality "a disease".
Critics have called the ruling regressive and medically unsound.
Brazil's Federal Council of Psychology banned psychologists from offering treatments that claim to change people's sexuality in 1999.
It said in a statement that the decision "opens the dangerous possibility of the use of sexual reversion therapies" and promised to fight it in court.
Prominent pop star Anitta shared her disgust in an online video, which has been watched by over a million people.
"That's what happens in my country. People dying, hungry, the government killing the country with corruption, no education, no hospitals, no opportunities... and the authorities are wasting their time to announce that homosexuality is a sickness," she said.
Ivete Sangolo, another of Brazil's most popular singers, wrote on Instagram: "The sick ones are those who believe in this great absurdity."
Her post has been liked over 236,000 times since it was posted a day ago.
On Twitter, many users shared support for the gay community, posting pictures and cartoons condemning the verdict.
"Of course, because in this country it is more important to 'cure' human sexuality than to treat prejudice," one man wrote in Portuguese.
"There's no cure for what is not a disease," others tweeted, using the hashtag #curagay ("gay cure").
That view was shared by Rogério Giannini, head of the Federal Council of Psychology, who told the UK's Guardian newspaper there was no medical basis for the alleged therapy.
"It is not a serious, academic debate, it is a debate connected to religious or conservative positions," he said.
Brazil is seeing growing push-back from conservative Christians over progressive policies which they see as an affront to traditional values.
They have protested against gay characters in TV soap operas, and got an art exhibition on gender and sexual diversity shut down last week.
Anger as Brazilian judge backs anti-gay 'therapy'
20 September 2017
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Brazilian activists and celebrities have condemned a court ruling that approves a "cure" for gay people.
