It's the first time any regulatory agency has classified glyphosate as carcinogenic, and the impact of California's decision could be far-reaching, and may be followed by other states and possibly other countries.
California's Office of Environmental Health Hazard Assessment has been responsible for the listing, although it doesn't restrict the sale or availability of Roundup.
Under the US's Proposition 65-more properly the Safe Drinking Water and Enforcement Act of 1986-chemicals that cause cancer, birth defects or other reproductive harm must be listed and published by the state.
The International Agency for Research on Cancer (IARC) described glyphosate as a "possible carcinogen" last March.
Studies among people who regularly use Roundup in their work-such as gardeners or farm workers-have discovered that it increases the risk for non-Hodgkin lymphoma, a cancer of the lymphatic system.
Roundup is sprayed on crops, parks, open spaces and gardens around the world.