Montagner, 54 and at the height of his career, was on a break from filming the hit series "Velho Chico" (Old Chico) in northeast Brazil when he went for a swim Thursday in the Sao Francisco River with co-star Camila Pitanga, according to the Globo media group, which broadcasts the show.
The pair was swept up in a strong current. Pitanga managed to grab onto a rock, but she was unable to keep hold of Montagner.
She cried out for help, but some locals apparently did not pay attention because they supposed filming was under way.
Montagner's body was found several hours later 30 meters (100 feet) underwater trapped among the riverbed rocks, the Globo group's flagship newspaper reported.
"Velho Chico," one of Brazil's most-watched "novelas" -- or primetime soaps -- tells the story of the power struggles between two families of landholders in the turbulent 1970s.
Tragically, Montagner played the role of Santo, who disappeared in the same river in a recent episode. In the series, he was rescued by an indigenous tribe.
Montagner's death triggered an outpouring of shock and sympathy in Brazil, where millions of viewers watch novelas each day.
The soap star's death might have been avoided: the riverside beach where the co-stars went swimming used to have "not safe for swimming" signs and lifeguards until 45 days ago, local media reported.
A public works project at the beach put up kiosks, a restaurant, playground and soccer area, but took down warning signs and fired its last lifeguards.
The actor, who was married with three children and started out in circus performing, made a relatively late start in television in 2008.
But he became massively popular nationwide thanks to his roles in a number of Globo network "novelas."
Media giant Globo produces more than 2,500 hours a year of programming -- a world record. Its soap operas and other series have been translated into more than 30 languages, and shown in more than 100 countries, company data show.
Filming in northeast Brazil has suspended and Globo sent in planes to fly out the production teams.
The fate of Montagner's last production at Globo was not immediately known.
