The 54-year-old had gone for a swim with an actress after a day of shooting in the north-eastern state of Sergipe.
She said he was dragged away by the river`s strong currents.
The actress, Camila Pitanga, cried for help but local people failed to act initially as they believed the drowning was a scene in the soap opera.
Authorities said there was little that anyone could have done.
In the soap opera, Montagner played a farmer who often fought off local gunmen. In one of those clashes, he was shot multiple times and appeared to drown in the same Sao Francisco river, but reappeared weeks later.
`Dangerous area`
"They thought they had chosen a safe spot to swim, but that is one of the most dangerous areas in the town of Caninde and usually avoided by locals," police chief Antonio Francisco Filho told O Globo.
One person, on average, has drowned every year in the town since 2005, reports said.
Four hours after the incident, rescue teams found Montagner`s body underwater, some 300 metres (984ft) from where he drowned.
`Life imitates art`
Montagner was born in Sao Paulo and had a career in the circus before becoming a television actor, in 2008.
This was his 12th role on TV. His first significant soap opera role was in 2011, and he had his film debut in 2012.
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