The Monsanto Company () was an American agrochemical and agricultural biotechnology corporation founded in 1901 and headquartered in Creve Coeur, Missouri. Monsanto's best known product is Roundup, a glyphosate-based herbicide, developed in the 1970s. Later, the company became a major producer of genetically engineered crops. In 2018, the company ranked 199th on the Fortune 500 of the largest United States corporations by revenue.

Monsanto was one of four groups to introduce genes into plants in 1983, and was among the first to conduct field trials of genetically modified crops in 1987. It was one of the top 10 US chemical companies until it divested most of its chemical businesses between 1997 and 2002, through a process of mergers and spin-offs that focused the company on biotechnology. Monsanto was one of the first companies to apply the biotechnology industry business model to agriculture, using techniques developed by biotech drug companies.^: 2–6 In this business model, companies recoup R&D expenses by exploiting biological patents.

Monsanto's roles in agricultural changes, biotechnology products, lobbying of government agencies, and roots as a chemical company have resulted in controversies. The company once manufactured controversial products such as the insecticide DDT, PCBs, Agent Orange, and recombinant bovine growth hormone. Its seed patenting model was criticized as biopiracy and a threat to biodiversity as invasive species.

In September 2016, German chemical company Bayer announced its intent to acquire Monsanto for US$66 billion in an all-cash deal. After gaining US and EU regulatory approval, the sale was completed on June 7, 2018. The name Monsanto was no longer used, but Monsanto's previous product brand names were maintained. In June 2020, Bayer agreed to pay numerous settlements in lawsuits involving ex-Monsanto products Roundup, PCBs and Dicamba.