A San Francisco jury has awarded $289 million (€253 million) to a former school groundsman who claimed Monsanto's popular Roundup weed killer contributed to his terminal cancer.
In its decision on Friday, the state court jury found the agribusiness giant failed to adequately warn of the risks of using Roundup.
Dewayne Johnson's lawyers said he sprayed Roundup and a similar product, Ranger Pro, in large quantities as a pest control manager at a San Francisco Bay Area school district.
He developed a rash and was 42 when he was diagnosed with non-Hodgkin's lymphoma in 2014.
Monsanto's lawyer, George Lombardi, said non-Hodgkin's lymphoma takes years to develop, so Johnson's cancer must have started before he worked for the district.
The company has denied ties between glyphosate, the active ingredient in Roundup, and cancer.
The lawsuit brought by Dewayne Johnson was the first to go to trial among hundreds filed in state and federal courts saying Roundup causes non-Hodgkin's lymphoma.
Jurors agreed the product contributed to Johnson's cancer and the company should have provided a label warning of the potential health hazard.
Johnson's lawyers sought and won $39 million (€34 million) in compensatory damages and $250 million (€218 million) of the $373 million (€326 million) they wanted in punitive damages.
"This jury found Monsanto acted with malice and oppression because they knew what they were doing was wrong and doing it with reckless disregard for human life," said Robert F. Kennedy Jr, a member of Johnson's legal team.
"This should send a strong message to the boardroom of Monsanto."
Monsanto says hundreds of studies have established that glyphosate is safe.
Spokesman Scott Partridge said the company would appeal, and said scientific studies and two government agencies had concluded that Roundup does not cause cancer.
"We are sympathetic to Mr Johnson and his family," Mr Partridge said.
"We will appeal this decision and continue to vigorously defend this product, which has a 40-year history of safe use and continues to be a vital, effective, and safe tool for farmers and others."
Johnson sprayed large quantities from a 50-gallon tank attached to a truck, and during gusty winds, the product would cover his face, said Brent Wisner, one of his lawyers.
Once, when a hose broke, the weed killer soaked his entire body.
Johnson read the label and even contacted the company after developing a rash but was never warned it could cause cancer, Wisner said.
"The simple fact is he is going to die. It's just a matter of time," Wisner told the jury in his opening statement last month.
The US Environmental Protection Agency says Roundup's active ingredient is safe for people when used in accordance with label directions.
However, the France-based International Agency for Research on Cancer, which is part of the World Health Organisation, classified it as a "probable human carcinogen" in 2015, and California added glyphosate to its list of chemicals known to cause cancer.