A federal jury on Wednesday ordered Monsanto to pay more than $80 million in damages to a California man whose cancer it determined was partly caused by his use of the popular weedkiller Roundup.

The six-member jury found that Monsanto should be held liable for the man’s illness because it failed to include a label on its product warning of the weedkiller’s risk of causing cancer.

The verdict, delivered in United States District Court in San Francisco, is a milestone in the continuing public debate over the health effects of Roundup and its active ingredient, glyphosate, the world’s most widely used weedkiller. Monsanto is currently defending itself against thousands of similar claims.

The plaintiff, Edwin Hardeman, 70, used Roundup to control weeds and poison oak on his property for 26 years. In 2015, he learned that he had non-Hodgkin’s lymphoma. The next year, after the World Health Organization’s International Agency for Research on Cancer declared glyphosate a probable carcinogen, Mr. Hardeman sued Monsanto.