Penicillin was discovered in 1928 by the Scottish physician and bacteriologist Alexander Fleming. He noticed that a mold (Penicillium rubens, previously identified as Penicillium notatum) growing on a petri dish inhibited the growth of Staphylococcus bacteria. Fleming called the antibacterial substance "penicillin" and published his findings in 1929.

Although Fleming discovered penicillin, he was initially unable to purify and mass-produce it. The development of penicillin as a widely used antibiotic required later contributions by scientists such as Howard Florey and Ernst Chain, who successfully isolated and manufactured penicillin on a large scale during World War II.

Fleming's discovery is recognized as a groundbreaking advancement in medicine, which led to the antibiotic era and saved millions of lives, earning him the Nobel Prize in Physiology or Medicine in 1945.