Chlorpyrifos (CPS), also known as Chlorpyrifos ethyl, is an organophosphate pesticide used on crops, animals, and buildings, and in other settings, to kill a number of pests, including insects and worms. It acts on the nervous systems of insects by inhibiting the acetylcholinesterase enzyme. Chlorpyrifos was patented in 1966 by Dow Chemical Company.

Chlorpyrifos is considered moderately hazardous to humans by the World Health Organization based on its acute toxicity. Exposure surpassing recommended levels has been linked to neurological effects, persistent developmental disorders, and autoimmune disorders. Exposure during pregnancy may harm the mental development of children, and most home uses of chlorpyrifos were banned in the U.S. in 2001. In agriculture, it is "one of the most widely used organophosphate insecticides" in the United States, and before being phased out for residential use it was one of the most used residential insecticides.

On 29 March 2017, EPA Administrator Scott Pruitt denied a petition to ban chlorpyrifos. However, on 9 August 2018, the U.S. 9th Circuit Court of Appeals ordered the EPA to ban the sale of chlorpyrifos in the United States within 60 days, though this ruling was almost immediately appealed by the Trump administration.

In May 2019 the California Department of Pesticide Regulation (CDPR) announced it will "cancel the registration that allows chlorpyrifos to be sold in California". In October 2019 CDPR announced an end to all chlorpyrifos sales by 6 February 2020. Possession and use of chlorpyrifos by agricultural growers were effectively banned in California by 31 December 2020.