In the 7-2 ruling, the justices sided with the US Fish and Wildlife Service, thwarting the Sierra Club's bid to obtain documents concerning a regulation finalized in 2014 relating to power plants. Barrett and the court's other five conservative justices were joined by liberal Justice Elena Kagan in the majority, with liberals Justices Stephen Breyer and Sonia Sotomayor in dissent.
The Senate approved Barrett for a lifetime job on the top US judicial body on October 26 after a contentious and accelerated confirmation process in the weeks before the Nov. 3 presidential election. She is one of three justices appointed by Republican former President Donald Trump and she replaced liberal Justice Ruther Bader Ginsburg, who died on September 18.
Trump touted Barrett's appointment during campaign rallies ahead of the election, which he lost to Democratic President Joe Biden. Her swift confirmation by the Senate, which at the time was controlled by Trump's fellow Republicans but is now led by the Democrats, moved the court further to the right and deprived Biden of an opportunity to replace Ginsburg with a liberal successor.
The ruling in the Sierra Club case limited the scope of US agency documents that would be subject to a federal law called the Freedom of Information Act, which lets people request certain government materials.
The group wanted access to internal documents concerning the Fish and Wildlife Service's conclusion that a proposed environmental regulation for cooling water intake structures that are used by power plants and other industrial facilities would not adversely affect endangered species, including fish, turtles and shellfish.
The agency initially found in 2013 that the regulation would put the species in jeopardy but its final recommendation to the Environmental Protection Agency in 2014 made the opposite conclusion.
Writing for the court, Barrett said the 2013 draft documents were protected from disclosure because "they reflect a preliminary view - not a final decision - about the likely effect of the EPA's proposed rule on endangered species."
A federal judge in California ruled in 2017 that 11 documents had to be disclosed. Trump's administration appealed and the San Francisco-based 9th US Circuit Court of Appeals in 2018 ruled partly for the government but still found that nine documents had to be released.
After the ruling, Sierra Club lawyer Elena Saxonhouse said the documents "were meant to - and did - determine subsequent agency actions" and therefore should have been disclosed.
"We're encouraged by the Supreme Court's affirmation that courts must inquire into the specific context of draft documents before allowing agencies to withhold them from the public," Saxonhouse added.
The case was argued the day before the election. It marked Barrett's first arguments as a justice. She previously served on a lower federal appeals court and as a legal scholar at the University of Notre Dame in Indiana.
So far, Barrett's biggest impact on the court came when she provided the decisive vote in favor of religious entities challenging COVID-19 restrictions in New York.