Some 200 workers were also found at another farm living in appalling conditions.
The site had no proper housing and inadequate food and sanitary conditions, said Marcelo Campos, an adviser at the Labour Ministry's special unit to monitor slavery in Brazil's vast interior.
"It’s the biggest find of this crime we’ve had since 1995," Campos said on Monday, referring to when the unit was created.
He said one worker died of a heart attack when the inspectors turned up at the farm where the 800 workers were found, about 70 of them seriously ill.
Another 200 workers were discovered at the second farm, which had a different owner, in the poor interior of the north-eastern Bahia state.
"It’s the biggest find of this crime we’ve had since 1995"
adviser to Labour Ministry
Despite government efforts, slave labourers often seized from isolated native communities have been used to clear space in Amazon rain forests for cattle ranches.
Before 1995, Brazil had no real policy to fight a practice that usually involves landlords hiring poor workers in a different region of the country and then transporting them thousands of kilometres to their isolated farms.
The workers are not paid and have no money to return to their homes. Sometimes they are prevented by armed guards from leaving the farms, where they are often not given proper food or housing.
The farm owners have so far not been charged, but Labour Ministry officials said they would team up with public prosecutors and try them for keeping workers in slave-like conditions.
If convicted, the owners could face up to four years in prison. Campos said the farmers would also have to pay the workers decent wages for their previous labours.
Since coming to power in January, Brazilian President Luiz Inacio Lula da Silva has pledged to end the practice. An estimated 25,000 people still live in slave-like conditions in the country that was the last to formally abolish slavery in the Americas, in 1888.