In a bold stride for the majority Roman Catholic nation, the National Council of Justice (NCJ), a panel which oversees Brazil's legal system and is headed by the chief justice of the Supreme Court, said government offices that issue marriage licenses had no standing to reject gay couples.
"This is the equivalent of authorizing same-sex marriage in Brazil," said Raquel Pereira de Castro Araujo, head of the human rights committee of the Brazilian bar association.
Supreme Court Chief Justice Joaquim Barbosa explained that there was no reason for government marriage licensing offices to wait for Congress to pass a law on same-sex marriage before extending gays rights they legally already have.
Barbosa noted that the Supreme Court in 2011 recognized stable homosexual unions, ruling that the constitution guaranteed them the same rights as heterosexual couples.
"Are we going to require the approval of a new law by Congress to put into effect the ruling that already has been made by the Supreme Court? That would make no sense," he said in comments quoted by the G1 news website.
The earlier Supreme Court decision "is binding" and should be followed by the lower courts, Barbosa stressed.
In Congress, a strong religious faction opposes same-sex marriage, and has not yet approved a law on same-sex marriage regulations. And the NCJ's decisions are subject to appeal before the Supreme Court.
Lawmaker Marco Feliciano, also an evangelical Christian pastor, said his Social Christian Party would appeal the change in the high Court.
"It's something most Brazilians do not want... it is unconstitutional. And it is invading in the turf of the legislative branch," Feliciano told AFP.
Since 2011 some offices have granted marriage licenses to gay couples and others have not.
While some state courts have recognized same-sex marriages, the council's ruling was the first to set out a national standard.
The ruling drew praise from same-sex marriage rights activists in Rio.
"The decision is timely and is the result of a long struggle of organized social movements," said Marjori Machi, president of the Rio de Janeiro Association of Transvestite and Transsexual People (ASTRA-RIO).
"This will reduce legal procedures and ensures more equality to Brazilian citizens. Before this there were first and second class citizens. Now all families will be respected," Machi told AFP.
Luiz Kignet, a family law specialist at PLKC Advogados in Sao Paulo, said a federal law on same-sex marriage does need to be on the books.
"The Court authority (NCJ) is not saying a (national, federal) law is not needed," Kignet said.
"But it is saying that same-sex marriage is constitutional even while the legislative body has not produced a law yet."
The move comes just over two months before Pope Francis attends World Youth Day in Brazil, the country with the most Roman Catholics in the world -- 123 million of its 194 million people.
Just last month Uruguay's legislature voted to allow same-sex marriages nationwide.
Uruguay's move came after Argentina approved gay marriage in 2010. Same-sex marriage has been permitted in Mexico City, but not the rest of Mexico, since 2009.
Roman Catholic church doctrine holds that marriage is for one man and one woman.