The Norwegian-born skier, who took gold at the Milano Cortina Games in February and won the last two giants of the 2025-26 season, was 0.24 of a second faster than the rest on a first run down Soelden's Rettenbach glacier piste that was shortened due to strong winds.
He went on to win by 0.48 from Switzerland's Loic Meillard, the Olympic slalom gold medallist. Five-time defending overall World Cup champion Marco Odermatt was third, 0.01 slower than his compatriot, after going fastest on the second run.
"What a relief to be able to open the show in this way," said Pinheiro Braathen, who is unbeaten in giant since his Olympic success. "I have worked so hard this whole summer.
"I've been searching for new curiosities, new paths to evolve from last year's Olympic gold and the (giant slalom crystal) globe. And you know, we're headed towards new goals, new objectives."
American River Radamus finished fourth from 15th after the first run, and Britain's Freddy Carrick-Smith went from bib number 46 to 21st and then 12th at the finish while some big names fell, including Italy's Giovanni Franzoni and Norway's Atle Lie McGrath.
American Paula Moltzan won the opening women's giant slalom on Saturday.
The men's World Cup season continues with a slalom in Levi, Finland, on November 15th, a day after the women compete there.
Pinheiro Braathen won last season's Levi slalom, his first victory with the Brazilian flag.