Former school groundskeeper Dewayne Johnson has been awarded almost $290 million after a jury at the Superior Court of California in San Francisco determined that "Roundup" played a "substantial factor" in his deteriorating health.
Two of Britain's leading garden centre chains are reviewing their products after a weedkiller was blamed for causing the terminal cancer of an American man awarded more than $289m (£226m) in damages.
Following eight weeks of trial proceedings, the San Francisco jury ordered Monsanto to pay US$250 million in punitive damages along with compensatory damages and other costs, bringing the total figure to almost US$290 million.
Johnson filed the lawsuit in 2016, alleging that Monsanto's Roundup and Ranger Pro herbicides are responsible for his cancer.
From "Agent Orange" and DDT to genetically modified crops, Monsanto has always been associated with controversial chemicals, but a USA court order for it to pay damages because one of its herbicides may cause cancer could open the door to thousands more claims against the company.
Monsanto always denied any link to the disease and said studies concluded the product was safe. While becoming the world's most popular and widely used herbicide, the question of whether it causes cancer has been hotly debated by environmentalists, regulators, researchers and lawyers - even as Monsanto has insisted for decades that it's perfectly safe.
Three years ago, doctors diagnosed Angie Dyer with a rare form of cancer - double hit large B-cell non-Hodgkin lymphoma.
"Monsanto made Roundup the oxycontin of pesticides and now the addiction and damage they caused have come home to roost", said Ken Cook, President of the Environmental Working Group, a USA environmental organization that researches toxic chemicals and advocates for corporate accountability.
The suit is one of hundreds in progress and will nearly certainly be appealed by Monsanto, which was recently purchased by chemical giant Bayer.
Johnson's team expressed confidence in the verdict, saying the judge in the case had kept out a mountain of more evidence backing their position.
Mr Johnson read the label but said he was never warned the product could cause cancer, Mr Wisner said.
The U.S. Environmental Protection Agency has gone back and forth on considering glyphosate a possible carcinogen.
George Lombardi, an attorney for the company, also argues that Johnson's illness takes years to develop, so it must have started well before he began working at the school district. That was enough for the World Health Organization to label the idea that glyphosate caused cancer as "probable".
Partridge said outside the courthouse that Monsanto had no intention of settling the slew of similar cases in the legal queue, saying if anything the verdict would prompt the company to work harder to demonstrate the weed killer is safe.
"The jury got it wrong", vice-president Scott Partridge said outside the courthouse in San Francisco.