A San Francisco court now says cases can be brought against the firm on the basis of testimony that Roundup is potentially a cause of the blood cancer non-Hodgkin's lymphoma.
Dozens of the suits were joined to be heard in the court of U.S. District Judge Vince Chhabria - who, even as he allowed the case to proceed, said the plaintiffs "appear to face a daunting challenge" in supporting their claims at the next phase of the case.
The company had told Chhabria in March that none of the plaintiffs' specialists fulfilled scientific or legal requirements for admissibility and urged the judge to dismiss the cases.
Lawsuits allege that Roundup maker Monsanto long knew about the cancer risk associated with the week killer but did not warn people.
"Moving forward, we will continue to defend these lawsuits with robust evidence that proves there is absolutely no connection between glyphosate and cancer", Partridge said in a statement.
Glyphosate is the active ingredient in Monsanto's Roundup herbicide, the most widely used weed-killer in the world. Farmers in California have been using the weed killer on more than 200 types of crops. The company faced more than 400 cancer lawsuits in the region and about 5,000 more nationwide.
Claims against Monsanto received a boost in 2015, when the International Agency for Research on Cancer - part of the World Health Organization - announced that two pesticides, including glyphosate, are "probably carcinogenic to humans". Homeowners use it on their lawns and gardens.
U.S courts decided glyphosate does most likely not cause cancer in humans last September.
Beate Ritz, an epidemiologist at the University of California, Los Angeles, testified for the plaintiffs that her review of scientific literature led her to conclude that glyphosate and glyphosate-based compounds such as Roundup can cause non-Hodgkin's lymphoma. Monsanto has attacked the global research agency's opinion as an outlier.
Monsanto introduced glyphosate in the market in the 1970s.