Bayer expressed disappointment with the decision, but maintained that the science confirmed that glyphosate-based herbicides do not cause cancer.
A California court on Tuesday found that Roundup was a "substantial factor" in Edwin Hardeman, 70, getting non-Hodgkin's lymphoma after spraying the weedkiller on his garden for decades.
The Hardeman's case is considered a landmark trial for hundreds of other plaintiffs in the U.S. with similar claims, which means the verdict could affect future litigation and other cancer patients and families.
The jurors found that Monsanto acted with "malice" and ordered the company to pay $289 million to Dewayne Johnson, who has two young sons and is in the end stages of his cancer, before the damages were reduced to $78.5 million.
He alleged that exposure to the weed killer led to his non-Hodgkin's lymphoma, a cancer that affects the immune system. "As similar lawsuits mount, the evidence will grow that Roundup is not safe, and that the company has tried to cover it up".
"We are confident the evidence in phase two will show that Monsanto's conduct has been appropriate and the company should not be liable for Mr. Hardeman's cancer", it said.
Pharmaceutical group Bayer had strongly rejected claims that its glyphosate-based Roundup product was carcinogenic.
Bayer AG shares dropped 13 per cent to 60.66 euros ($68.84) in Frankfurt Wednesday.
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Bayer has denied its products cause cancer.
Last August, a California jury in state court delivered a historic verdict by ruling that Roundup caused the terminal cancer of a former school groundskeeper.
The European Food Safety Authority (EFSA) also says glyphosate is unlikely to cause cancer in humans.
The latest case has so far brought no damages award against Bayer, as jurors now have to decide whether Monsanto is liable for the harm suffered by Hardeman.
Litzenburg said he and other attorneys have more than 4,000 similar cases awaiting trial in various state courts.
Brokerage Warburg lowered its recommendation to "Hold" from "Buy", arguing that the with the renewed setback upcoming glyphosate court cases would remain a drag on the share price.
A wave of lawsuits has put pressure on Bayer since its $63-billion takeover of Monsanto past year, spooking investors who worry damages payouts could quickly mount into the billions if the firm fails to convince courts its product is safe. "Now we can focus on the evidence that Monsanto has not taken a responsible, objective approach to the safety of Roundup", Aimee Wagstaff and Jennifer Moore, Hardeman's attorneys, said in a joint statement. Even as more cases are waiting to be heard in United States courts, more cancer sufferers are also queuing to file suits against Monsanto. "Among the four World Health Organization agencies that have evaluated the safety of glyphosate, IARC is the only World Health Organization entity to find an association between glyphosate and carcinogenicity", it said in a statement.