"The disparagement clause denies registration to any mark that is offensive to a substantial percentage of the members of any group", Alito writes.
The PTO denied the trademark citing the Lanham Act, a 71-year-old law that, in part, prohibits trademark protection for offensive names.
Today's ruling by the U.S. Supreme Court, that even trademarks which are racially disparaging are protected by the First Amendment, and therefore such trademarks are entitled to federal registration, nearly certainly dooms a legal action by Native Americans to cancel several "R*dsk*n" trademarks.
However, the name "R*dsk*in" was not adopted by Native Americans, and it has been found to be a racially disparaging slur by virtually every Native American organization.
Redskins owner Dan Snyder said he was "thrilled" with the Supreme Court's ruling, and team attorney Lisa Blatt said the court's decision effectively resolves the Redskins' longstanding dispute with the government.
Saudi Arabia says airspace ban on Qatari flights to protect citizens
At the heart of the dispute are the long-standing allegations linking Qatar to regional Islamist and militant groups. Qatar's neighbors cut ties with the country over accusations that it sponsors Islamic extremism, which Qatar denies.
The judgment was unanimous - with two asterisks: Justice Neil Gorsuch, who wasn't yet on the court when the case was argued in January, did not take part.
But while the Slants and Washington NFL team have the Lanham Trademark Act in common, their cases differ in other ways - most notably, the Washington NFL team is not made up of Native Americans who are trying to reclaim an offensive name.
The Supreme Court says the government can't refuse to register trademarks that are considered offensive.
In what D.C.'s football team is hailing as a victory for the disputed Redskins name, the Supreme Court ruled today that trademarks can't be denied on the basis of the trademark office ruling that the term is disparaging.
The outcome is likely to affect the legal case of the Washington Redskins, whose trademark registration was revoked in 2014 under the same disparagement clause.
"We're beyond humbled and thrilled to have won this case at the Supreme Court", Tam wrote in a Facebook post.
Tam headed to federal court years ago after he was unable to obtain a trademark.
"Holding that the registration of a trademark converts the mark into government speech would constitute a huge and unsafe extension of the government-speech doctrine, for other systems of government registration [such as copyright] could easily be characterized in the same way", Justice Samuel Alito wrote in the majority opinion.