In a landmark case, a US jury found that Monsanto knew its Roundup and RangerPro weedkillers were dangerous and failed to warn consumers.
It is the first lawsuit to go to trial alleging a glyphosate link to cancer.
The claimant in the case, groundskeeper DeWayne Johnson, is among more than 5,000 similar plaintiffs across the US.
According to his lawyers, Johnson was diagnosed with non-Hodgkin's lymphoma in 2014 after he regularly used a form of RangerPro while working at a school in Benicia, California.
Jurors found on Friday that the company had acted with "malice" and that its weedkillers contributed "substantially" to Johnson's terminal illness.
Johnson's lawyer, Brent Wisner, said the jury's verdict showed the evidence was "overwhelming".
"When you are right, it is really easy to win," he said, adding that the ruling was just "the tip of the spear" of future legal cases.
Monsanto, however, denied that glyphosate causes cancer and said it will appeal the ruling.
"The jury got it wrong," Monsanto Vice-President Scott Partridge said outside the courthouse in San Francisco.
Monsanto said in a statement that it was "sympathetic to Mr Johnson and his family" but it would "continue to vigorously defend this product, which has a 40-year history of safe use" while also mentioning the US Environmental Protection Agency or EPA, the US National Institutes of Health and regulatory authorities around the world - supported the fact that glyphosate does not cause cancer.
Correspondents say the California ruling is likely to lead to hundreds of other claims against Monsanto, which was recently bought by the German conglomerate Bayer AG.
Glyphosate is the world's most common weedkiller and the science about its safety is still far from settled.
In 2015 the International Agency for Research on Cancer, the World Health Organisation’s cancer agency concluded that it was “probably carcinogenic to human humans”. But the EPA continues to insist that glyphosate is safe when used carefully.