US District Judge Vince Chhabria announced his ruling in San Francisco on Monday.
Chhabria reduced the amount that Germany's Bayer AG has to pay the claimant to $25.27 million (€22.4 million). Bayer bought Roundup maker Monsanto for $63 billion last year.
Following a four-week trial in March a federal jury awarded $5 million (€4.4 million) in compensatory and $75 million in punitive damages to a man who blamed his cancer on glyphosate-based weed killer Roundup.
Edwin Hardeman was diagnosed with non-Hodgkin's lymphoma in 2014.
Limits on ratios for damages
At a hearing to discuss Bayer's request to overturn the verdict earlier this month, Chhabria: said "It's quite clear that under the Constitution I'm required to reduce the punitive damages award and it's just a question of how much."
US Supreme Court rulings limit the ratio of punitive to compensatory damages to 9 to 1.
The judge said he would also take into account the fact that Hardeman was now in full remission and unlikely to suffer as much as he had in the past.
Bayer says Roundup — and its active ingredient glyphosate — are safe for human use and not carcinogenic. However, in 2015 the World Health Organization's cancer arm reached a different conclusion, classifying glyphosate as "probably carcinogenic to humans."
Litigation hits share price
The Leverkusen-based company is facing lawsuits from more than 13,400 plaintiffs in the US.
Earlier this year, Bayer CEO Werner Baumann conceded the effect the court cases had had on the share price: "We have lost two cases in lower courts. That is why the company is massively affected. You see it in our share price," he said at an academic business event in Cologne in April. "You see it selectively, mainly here in Germany and in France — less so in the USA — in our reputational scores."
Over the past year, Bayer's share price has dropped dramatically and the company has seen €30 billion wiped off its market value since the first court ruling last August. Its share price has fallen from €96 in August 2018 to €58.63 at close of markets in Frankfurt on Monday.