Bayer AG (German pronunciation: [ˈbaɪɐ]) is a chemical and pharmaceutical company founded in Barmen (today a part of Wuppertal), Germany in 1863. It is headquartered in Leverkusen, North Rhine-Westphalia, Germany and well known for its original brand of aspirin.

Bayer AG was founded in Barmen (today a part of Wuppertal), Germany in 1863 by Friedrich Bayer and his partner, Johann Friedrich Weskott.

Bayer's first major product was acetylsalicylic acid (originally discovered by French chemist Charles Frederic Gerhardt in 1853), a modification of salicylic acid or salicin, a folk remedy found in the bark of the willow plant. By 1899, Bayer's trademark Aspirin was registered worldwide for Bayer's brand of acetylsalicylic acid, but because of the confiscation of Bayer's US assets and trademarks during World War I by the United States — and the subsequent widespread usage of the word to describe all brands of the compound —, "Aspirin" lost its trademark status in the United States, France, and the United Kingdom. It is now widely used in the US, UK, and France for all brands of the drug. However in over 80 other countries, such as Canada, Mexico, Germany, and Switzerland, it is still a registered trademark of Bayer.